text: """Module to handle the operations within the aggregate pipeline."""
  
  import bisect
  import collections
  import copy
  import datetime
  import decimal
  import itertools
  import math
  import numbers
  import random
  import re
  import warnings
  
  from sentinels import NOTHING
  import six
  from six import moves, raise_from
  
  from mongomock import command_cursor
  from mongomock import filtering
  from mongomock import helpers
  from mongomock import OperationFailure
  
  try:
      from bson.errors import InvalidDocument
      from bson import Regex, decimal128
      decimal_support = True
      _RE_TYPES = (helpers.RE_TYPE, Regex)
  except ImportError:
      InvalidDocument = OperationFailure
      decimal_support = False
      _RE_TYPES = (helpers.RE_TYPE)
  
  _random = random.Random()
  
  
  group_operators = [
      '$addToSet',
      '$avg',
      '$first',
      '$last',
      '$max',
      '$mergeObjects',
      '$min',
      '$push',
      '$stdDevPop',
      '$stdDevSamp',
      '$sum',
  ]
  arithmetic_operators = [
      '$abs',
      '$add',
      '$ceil',
      '$divide',
      '$exp',
      '$floor',
      '$ln',
      '$log',
      '$log10',
      '$mod',
      '$multiply',
      '$pow',
      '$sqrt',
      '$subtract',
      '$trunc',
  ]
  project_operators = [
      '$max',
      '$min',
      '$avg',
      '$sum',
      '$stdDevPop',
      '$stdDevSamp',
      '$arrayElemAt',
  ]
  control_flow_operators = [
      '$switch',
  ]
  projection_operators = ['$map', '$let', '$literal']
  date_operators = [
      '$dateFromString',
      '$dateToString',
      '$dayOfMonth',
      '$dayOfWeek',
      '$dayOfYear',
      '$hour',
      '$isoDayOfWeek',
      '$isoWeek',
      '$isoWeekYear',
      '$millisecond',
      '$minute',
      '$month',
      '$second',
      '$week',
      '$year',
  ]
  conditional_operators = ['$cond', '$ifNull']
  array_operators = [
      '$concatArrays',
      '$filter',
      '$indexOfArray',
      '$isArray',
      '$range',
      '$reduce',
      '$reverseArray',
      '$size',
      '$slice',
      '$zip',
  ]
  object_operators = [
      '$mergeObjects',
  ]
  text_search_operators = ['$meta']
  string_operators = [
      '$concat',
      '$indexOfBytes',
      '$indexOfCP',
      '$regexMatch',
      '$split',
      '$strcasecmp',
      '$strLenBytes',
      '$strLenCP',
      '$substr',
      '$substrBytes',
      '$substrCP',
      '$toLower',
      '$toUpper',
      '$trim',
  ]
  comparison_operators = [
      '$cmp',
      '$eq',
      '$ne',
  ] + list(filtering.SORTING_OPERATOR_MAP.keys())
  boolean_operators = ['$and', '$or', '$not']
  set_operators = [
      '$in',
      '$setEquals',
      '$setIntersection',
      '$setDifference',
      '$setUnion',
      '$setIsSubset',
      '$anyElementTrue',
      '$allElementsTrue',
  ]
  
  type_convertion_operators = [
      '$convert',
      '$toString',
      '$toInt',
      '$toDecimal',
      '$toLong',
      '$arrayToObject'
  ]
  
  
  def _avg_operation(values):
      values_list = list(v for v in values if isinstance(v, numbers.Number))
      if not values_list:
          return None
      return sum(values_list) / float(len(list(values_list)))
  
  
  def _group_operation(values, operator):
      values_list = list(v for v in values if v is not None)
      if not values_list:
          return None
      return operator(values_list)
  
  
  def _sum_operation(values):
      values_list = list()
      if decimal_support:
          for v in values:
              if isinstance(v, numbers.Number):
                  values_list.append(v)
              elif isinstance(v, decimal128.Decimal128):
                  values_list.append(v.to_decimal())
      else:
          values_list = list(v for v in values if isinstance(v, numbers.Number))
      sum_value = sum(values_list)
      return decimal128.Decimal128(sum_value) if isinstance(sum_value, decimal.Decimal) else sum_value
  
  
  def _merge_objects_operation(values):
      merged_doc = dict()
      for v in values:
          if isinstance(v, dict):
              merged_doc.update(v)
      return merged_doc
  
  
  _GROUPING_OPERATOR_MAP = {
      '$sum': _sum_operation,
      '$avg': _avg_operation,
      '$mergeObjects': _merge_objects_operation,
      '$min': lambda values: _group_operation(values, min),
      '$max': lambda values: _group_operation(values, max),
  }
  
  
  class _Parser(object):
      """Helper to parse expressions within the aggregate pipeline."""
  
      def __init__(self, doc_dict, user_vars=None, ignore_missing_keys=False):
          self._doc_dict = doc_dict
          self._ignore_missing_keys = ignore_missing_keys
          self._user_vars = user_vars or {}
  
      def parse(self, expression):
          """Parse a MongoDB expression."""
          if not isinstance(expression, dict):
              # May raise a KeyError despite the ignore missing key.
              return self._parse_basic_expression(expression)
  
          if len(expression) > 1 and any(key.startswith('$') for key in expression):
              raise OperationFailure(
                  'an expression specification must contain exactly one field, '
                  'the name of the expression. Found %d fields in %s'
                  % (len(expression), expression))
  
          value_dict = {}
          for k, v in six.iteritems(expression):
              if k in arithmetic_operators:
                  return self._handle_arithmetic_operator(k, v)
              if k in project_operators:
                  return self._handle_project_operator(k, v)
              if k in projection_operators:
                  return self._handle_projection_operator(k, v)
              if k in comparison_operators:
                  return self._handle_comparison_operator(k, v)
              if k in date_operators:
                  return self._handle_date_operator(k, v)
              if k in array_operators:
                  return self._handle_array_operator(k, v)
              if k in conditional_operators:
                  return self._handle_conditional_operator(k, v)
              if k in control_flow_operators:
                  return self._handle_control_flow_operator(k, v)
              if k in set_operators:
                  return self._handle_set_operator(k, v)
              if k in string_operators:
                  return self._handle_string_operator(k, v)
              if k in type_convertion_operators:
                  return self._handle_type_convertion_operator(k, v)
              if k in boolean_operators:
                  return self._handle_boolean_operator(k, v)
              if k in text_search_operators + projection_operators + object_operators:
                  raise NotImplementedError(
                      "'%s' is a valid operation but it is not supported by Mongomock yet." % k)
              if k.startswith('$'):
                  raise OperationFailure("Unrecognized expression '%s'" % k)
              try:
                  value = self.parse(v)
              except KeyError:
                  if self._ignore_missing_keys:
                      continue
                  raise
              value_dict[k] = value
  
          return value_dict
  
      def parse_many(self, values):
          for value in values:
              try:
                  yield self.parse(value)
              except KeyError:
                  if self._ignore_missing_keys:
                      yield None
                  else:
                      raise
  
      def _parse_to_bool(self, expression):
          """Parse a MongoDB expression and then convert it to bool"""
          # handles converting `undefined` (in form of KeyError) to False
          try:
              return helpers.mongodb_to_bool(self.parse(expression))
          except KeyError:
              return False
  
      def _parse_or_nothing(self, expression):
          try:
              return self.parse(expression)
          except KeyError:
              return NOTHING
  
      def _parse_basic_expression(self, expression):
          if isinstance(expression, six.string_types) and expression.startswith('$'):
              if expression.startswith('$$'):
                  return helpers.get_value_by_dot(dict({
                      'ROOT': self._doc_dict,
                      'CURRENT': self._doc_dict,
                  }, **self._user_vars), expression[2:])
              return helpers.get_value_by_dot(self._doc_dict, expression[1:], can_generate_array=True)
          return expression
  
      def _handle_boolean_operator(self, operator, values):
          if operator == '$and':
              return all([self._parse_to_bool(value) for value in values])
          if operator == '$or':
              return any(self._parse_to_bool(value) for value in values)
          if operator == '$not':
              return not self._parse_to_bool(values)
          # This should never happen: it is only a safe fallback if something went wrong.
          raise NotImplementedError(  # pragma: no cover
              "Although '%s' is a valid boolean operator for the "
              'aggregation pipeline, it is currently not implemented'
              ' in Mongomock.' % operator
          )
  
      def _handle_arithmetic_operator(self, operator, values):
          if operator == '$abs':
              return abs(self.parse(values))
          if operator == '$add':
              return sum(self.parse(value) for value in values)
          if operator == '$ceil':
              return math.ceil(self.parse(values))
          if operator == '$divide':
              assert len(values) == 2, 'divide must have only 2 items'
              return self.parse(values[0]) / self.parse(values[1])
          if operator == '$exp':
              return math.exp(self.parse(values))
          if operator == '$floor':
              return math.floor(self.parse(values))
          if operator == '$ln':
              return math.log(self.parse(values))
          if operator == '$log':
              assert len(values) == 2, 'log must have only 2 items'
              return math.log(self.parse(values[0]), self.parse(values[1]))
          if operator == '$log10':
              return math.log10(self.parse(values))
          if operator == '$mod':
              assert len(values) == 2, 'mod must have only 2 items'
              return math.fmod(self.parse(values[0]), self.parse(values[1]))
          if operator == '$multiply':
              return moves.reduce(
                  lambda x, y: x * y,
                  (self.parse(value) for value in values))
          if operator == '$pow':
              assert len(values) == 2, 'pow must have only 2 items'
              return math.pow(self.parse(values[0]), self.parse(values[1]))
          if operator == '$sqrt':
              return math.sqrt(self.parse(values))
          if operator == '$subtract':
              assert len(values) == 2, 'subtract must have only 2 items'
              value_0 = self.parse(values[0])
              value_1 = self.parse(values[1])
              if isinstance(value_0, datetime.datetime) and \
                      isinstance(value_1, (six.integer_types, float)):
                  value_1 = datetime.timedelta(milliseconds=value_1)
              res = value_0 - value_1
              if isinstance(res, datetime.timedelta):
                  return round(res.total_seconds() * 1000)
              return res
          if operator == '$trunc':
              return math.trunc(self.parse(values))
          # This should never happen: it is only a safe fallback if something went wrong.
          raise NotImplementedError(  # pragma: no cover
              "Although '%s' is a valid aritmetic operator for the aggregation "
              'pipeline, it is currently not implemented  in Mongomock.' % operator)
  
      def _handle_project_operator(self, operator, values):
          if operator in _GROUPING_OPERATOR_MAP:
              values = self.parse(values) if isinstance(values, str) else self.parse_many(values)
              return _GROUPING_OPERATOR_MAP[operator](values)
          if operator == '$arrayElemAt':
              key, index = values
              array = self._parse_basic_expression(key)
              index = self.parse(index)
              return array[index]
          raise NotImplementedError("Although '%s' is a valid project operator for the "
                                    'aggregation pipeline, it is currently not implemented '
                                    'in Mongomock.' % operator)
  
      def _handle_projection_operator(self, operator, value):
          if operator == '$literal':
              return value
          if operator == '$let':
              if not isinstance(value, dict):
                  raise InvalidDocument('$let only supports an object as its argument')
              for field in ('vars', 'in'):
                  if field not in value:
                      raise OperationFailure("Missing '{}' parameter to $let".format(field))
              if not isinstance(value['vars'], dict):
                  raise OperationFailure('invalid parameter: expected an object (vars)')
              user_vars = {
                  var_key: self.parse(var_value)
                  for var_key, var_value in six.iteritems(value['vars'])
              }
              return _Parser(
                  self._doc_dict,
                  dict(self._user_vars, **user_vars),
                  ignore_missing_keys=self._ignore_missing_keys,
              ).parse(value['in'])
          raise NotImplementedError("Although '%s' is a valid project operator for the "
                                    'aggregation pipeline, it is currently not implemented '
                                    'in Mongomock.' % operator)
  
      def _handle_comparison_operator(self, operator, values):
          assert len(values) == 2, 'Comparison requires two expressions'
          a = self.parse(values[0])
          b = self.parse(values[1])
          if operator == '$eq':
              return a == b
          if operator == '$ne':
              return a != b
          if operator in filtering.SORTING_OPERATOR_MAP:
              return filtering.bson_compare(filtering.SORTING_OPERATOR_MAP[operator], a, b)
          raise NotImplementedError(
              "Although '%s' is a valid comparison operator for the "
              'aggregation pipeline, it is currently not implemented '
              ' in Mongomock.' % operator)
  
      def _handle_string_operator(self, operator, values):
          if operator == '$toLower':
              parsed = self.parse(values)
              return str(parsed).lower() if parsed is not None else ''
          if operator == '$toUpper':
              parsed = self.parse(values)
              return str(parsed).upper() if parsed is not None else ''
          if operator == '$concat':
              parsed_list = list(self.parse_many(values))
              return None if None in parsed_list else ''.join([str(x) for x in parsed_list])
          if operator == '$split':
              if len(values) != 2:
                  raise OperationFailure('split must have 2 items')
              try:
                  string = self.parse(values[0])
                  delimiter = self.parse(values[1])
              except KeyError:
                  return None
  
              if string is None or delimiter is None:
                  return None
              if not isinstance(string, six.string_types):
                  raise TypeError('split first argument must evaluate to string')
              if not isinstance(delimiter, six.string_types):
                  raise TypeError('split second argument must evaluate to string')
              return string.split(delimiter)
          if operator == '$substr':
              if len(values) != 3:
                  raise OperationFailure('substr must have 3 items')
              string = str(self.parse(values[0]))
              first = self.parse(values[1])
              length = self.parse(values[2])
              if string is None:
                  return ''
              if first < 0:
                  warnings.warn('Negative starting point given to $substr is accepted only until '
                                'MongoDB 3.7. This behavior will change in the future.')
                  return ''
              if length < 0:
                  warnings.warn('Negative length given to $substr is accepted only until '
                                'MongoDB 3.7. This behavior will change in the future.')
              second = len(string) if length < 0 else first + length
              return string[first:second]
          if operator == '$strcasecmp':
              if len(values) != 2:
                  raise OperationFailure('strcasecmp must have 2 items')
              a, b = str(self.parse(values[0])), str(self.parse(values[1]))
              return 0 if a == b else -1 if a < b else 1
          if operator == '$regexMatch':
              if not isinstance(values, dict):
                  raise OperationFailure(
                      '$regexMatch expects an object of named arguments but found: %s' % type(values))
              for field in ('input', 'regex'):
                  if field not in values:
                      raise OperationFailure("$regexMatch requires '%s' parameter" % field)
              unknown_args = set(values) - {'input', 'regex', 'options'}
              if unknown_args:
                  raise OperationFailure(
                      '$regexMatch found an unknown argument: %s' % list(unknown_args)[0])
  
              try:
                  input_value = self.parse(values['input'])
              except KeyError:
                  return False
              if not isinstance(input_value, six.string_types):
                  raise OperationFailure("$regexMatch needs 'input' to be of type string")
  
              try:
                  regex_val = self.parse(values['regex'])
              except KeyError:
                  return False
              options = None
              for option in values.get('options', ''):
                  if option not in 'imxs':
                      raise OperationFailure(
                          '$regexMatch invalid flag in regex options: %s' % option)
                  re_option = getattr(re, option.upper())
                  if options is None:
                      options = re_option
                  else:
                      options |= re_option
              if isinstance(regex_val, str):
                  if options is None:
                      regex = re.compile(regex_val)
                  else:
                      regex = re.compile(regex_val, options)
              elif 'options' in values and regex_val.flags:
                  raise OperationFailure(
                      "$regexMatch: regex option(s) specified in both 'regex' and 'option' fields")
              elif isinstance(regex_val, helpers.RE_TYPE):
                  if options and not regex_val.flags:
                      regex = re.compile(regex_val.pattern, options)
                  elif regex_val.flags & ~(re.I | re.M | re.X | re.S):
                      raise OperationFailure(
                          '$regexMatch invalid flag in regex options: %s' % regex_val.flags)
                  else:
                      regex = regex_val
              elif isinstance(regex_val, _RE_TYPES):
                  # bson.Regex
                  if regex_val.flags & ~(re.I | re.M | re.X | re.S):
                      raise OperationFailure(
                          '$regexMatch invalid flag in regex options: %s' % regex_val.flags)
                  regex = re.compile(regex_val.pattern, regex_val.flags or options)
              else:
                  raise OperationFailure("$regexMatch needs 'regex' to be of type string or regex")
  
              return bool(regex.search(input_value))
  
          # This should never happen: it is only a safe fallback if something went wrong.
          raise NotImplementedError(  # pragma: no cover
              "Although '%s' is a valid string operator for the aggregation "
              'pipeline, it is currently not implemented  in Mongomock.' % operator)
  
      def _handle_date_operator(self, operator, values):
          out_value = self.parse(values)
          if operator == '$dayOfYear':
              return out_value.timetuple().tm_yday
          if operator == '$dayOfMonth':
              return out_value.day
          if operator == '$dayOfWeek':
              return (out_value.isoweekday() % 7) + 1
          if operator == '$year':
              return out_value.year
          if operator == '$month':
              return out_value.month
          if operator == '$week':
              return int(out_value.strftime('%U'))
          if operator == '$hour':
              return out_value.hour
          if operator == '$minute':
              return out_value.minute
          if operator == '$second':
              return out_value.second
          if operator == '$millisecond':
              return int(out_value.microsecond / 1000)
          if operator == '$dateToString':
              if not isinstance(values, dict):
                  raise OperationFailure(
                      '$dateToString operator must correspond a dict'
                      'that has "format" and "date" field.'
                  )
              if not isinstance(values, dict) or not {'format', 'date'} <= set(values):
                  raise OperationFailure(
                      '$dateToString operator must correspond a dict'
                      'that has "format" and "date" field.'
                  )
              if '%L' in out_value['format']:
                  raise NotImplementedError(
                      'Although %L is a valid date format for the '
                      '$dateToString operator, it is currently not implemented '
                      ' in Mongomock.'
                  )
              if 'onNull' in values:
                  raise NotImplementedError(
                      'Although onNull is a valid field for the '
                      '$dateToString operator, it is currently not implemented '
                      ' in Mongomock.'
                  )
              if 'timezone' in values.keys():
                  raise NotImplementedError(
                      'Although timezone is a valid field for the '
                      '$dateToString operator, it is currently not implemented '
                      ' in Mongomock.'
                  )
              return out_value['date'].strftime(out_value['format'])
  
          raise NotImplementedError(
              "Although '%s' is a valid date operator for the "
              'aggregation pipeline, it is currently not implemented '
              ' in Mongomock.' % operator)
  
      def _handle_array_operator(self, operator, value):
          if operator == '$concatArrays':
              if not isinstance(value, (list, tuple)):
                  value = [value]
  
              parsed_list = list(self.parse_many(value))
              for parsed_item in parsed_list:
                  if parsed_item is not None and not isinstance(parsed_item, (list, tuple)):
                      raise OperationFailure(
                          '$concatArrays only supports arrays, not {}'.format(type(parsed_item))
                      )
  
              return None if None in parsed_list else list(itertools.chain.from_iterable(parsed_list))
  
          if operator == '$size':
              if isinstance(value, list):
                  if len(value) != 1:
                      raise OperationFailure('Expression $size takes exactly 1 arguments. '
                                             '%d were passed in.' % len(value))
                  value = value[0]
              array_value = self._parse_or_nothing(value)
              if not isinstance(array_value, (list, tuple)):
                  raise OperationFailure(
                      'The argument to $size must be an array, but was of type: %s' %
                      ('missing' if array_value is NOTHING else type(array_value)))
              return len(array_value)
  
          if operator == '$filter':
              if not isinstance(value, dict):
                  raise OperationFailure('$filter only supports an object as its argument')
              extra_params = set(value) - {'input', 'cond', 'as'}
              if extra_params:
                  raise OperationFailure('Unrecognized parameter to $filter: %s' % extra_params.pop())
              missing_params = {'input', 'cond'} - set(value)
              if missing_params:
                  raise OperationFailure("Missing '%s' parameter to $filter" % missing_params.pop())
  
              input_array = self.parse(value['input'])
              fieldname = value.get('as', 'this')
              cond = value['cond']
              return [
                  item for item in input_array
                  if _Parser(
                      self._doc_dict,
                      dict(self._user_vars, **{fieldname: item}),
                      ignore_missing_keys=self._ignore_missing_keys,
                  ).parse(cond)
              ]
          if operator == '$slice':
              if not isinstance(value, list):
                  raise OperationFailure('$slice only supports a list as its argument')
              if len(value) < 2 or len(value) > 3:
                  raise OperationFailure('Expression $slice takes at least 2 arguments, and at most '
                                         '3, but {} were passed in'.format(len(value)))
              array_value = self.parse(value[0])
              if not isinstance(array_value, list):
                  raise OperationFailure(
                      'First argument to $slice must be an array, but is of type: {}'
                      .format(type(array_value)))
              for num, v in zip(('Second', 'Third'), value[1:]):
                  if not isinstance(v, six.integer_types):
                      raise OperationFailure(
                          '{} argument to $slice must be numeric, but is of type: {}'
                          .format(num, type(v)))
              if len(value) > 2 and value[2] <= 0:
                  raise OperationFailure('Third argument to $slice must be '
                                         'positive: {}'.format(value[2]))
  
              start = value[1]
              if start < 0:
                  if len(value) > 2:
                      stop = len(array_value) + start + value[2]
                  else:
                      stop = None
              elif len(value) > 2:
                  stop = start + value[2]
              else:
                  stop = start
                  start = 0
              return array_value[start:stop]
  
          raise NotImplementedError(
              "Although '%s' is a valid array operator for the "
              'aggregation pipeline, it is currently not implemented '
              'in Mongomock.' % operator)
  
      def _handle_type_convertion_operator(self, operator, values):
          if operator == '$toString':
              try:
                  parsed = self.parse(values)
              except KeyError:
                  return None
              if isinstance(parsed, bool):
                  return str(parsed).lower()
              if isinstance(parsed, datetime.datetime):
                  return parsed.isoformat()[:-3] + 'Z'
              return str(parsed)
  
          if operator == '$toInt':
              try:
                  parsed = self.parse(values)
              except KeyError:
                  return None
              if decimal_support:
                  if isinstance(parsed, decimal128.Decimal128):
                      return int(parsed.to_decimal())
                  return int(parsed)
              raise NotImplementedError(
                  'You need to import the pymongo library to support decimal128 type.'
              )
  
          if operator == '$toLong':
              try:
                  parsed = self.parse(values)
              except KeyError:
                  return None
              if decimal_support:
                  if isinstance(parsed, decimal128.Decimal128):
                      return helpers.to_long(parsed.to_decimal())
                  return helpers.to_long(parsed)
              raise NotImplementedError(
                  'You need to import the pymongo library to support decimal128 type.'
              )
  
          # Document: https://docs.mongodb.com/manual/reference/operator/aggregation/toDecimal/
          if operator == '$toDecimal':
              if not decimal_support:
                  raise NotImplementedError(
                      'You need to import the pymongo library to support decimal128 type.'
                  )
              try:
                  parsed = self.parse(values)
              except KeyError:
                  return None
              if isinstance(parsed, bool):
                  parsed = '1' if parsed is True else '0'
                  decimal_value = decimal128.Decimal128(parsed)
              elif isinstance(parsed, int):
                  decimal_value = decimal128.Decimal128(str(parsed))
              elif isinstance(parsed, float):
                  exp = decimal.Decimal('.00000000000000')
                  decimal_value = decimal.Decimal(str(parsed)).quantize(exp)
                  decimal_value = decimal128.Decimal128(decimal_value)
              elif isinstance(parsed, decimal128.Decimal128):
                  decimal_value = parsed
              elif isinstance(parsed, str):
                  try:
                      decimal_value = decimal128.Decimal128(parsed)
                  except decimal.InvalidOperation as err:
                      raise_from(OperationFailure(
                          "Failed to parse number '%s' in $convert with no onError value:"
                          'Failed to parse string to decimal' % parsed), err)
              elif isinstance(parsed, datetime.datetime):
                  epoch = datetime.datetime.utcfromtimestamp(0)
                  string_micro_seconds = str((parsed - epoch).total_seconds() * 1000).split('.')[0]
                  decimal_value = decimal128.Decimal128(string_micro_seconds)
              else:
                  raise TypeError("'%s' type is not supported" % type(parsed))
              return decimal_value
  
          # Document: https://docs.mongodb.com/manual/reference/operator/aggregation/arrayToObject/
          if operator == '$arrayToObject':
              try:
                  parsed = self.parse(values)
              except KeyError:
                  return None
  
              if parsed is None:
                  return None
  
              if not isinstance(parsed, (list, tuple)):
                  raise OperationFailure(
                      '$arrayToObject requires an array input, found: {}'.format(type(parsed))
                  )
  
              if all(isinstance(x, dict) and set(x.keys()) == {'k', 'v'} for x in parsed):
                  return {d['k']: d['v'] for d in parsed}
  
              if all(isinstance(x, (list, tuple)) and len(x) == 2 for x in parsed):
                  return dict(parsed)
  
              raise OperationFailure(
                  'arrays used with $arrayToObject must contain documents '
                  'with k and v fields or two-element arrays'
              )
  
          raise NotImplementedError(
              "Although '%s' is a valid type conversion operator for the "
              'aggregation pipeline, it is currently not implemented '
              'in Mongomock.' % operator)
  
      def _handle_conditional_operator(self, operator, values):
          if operator == '$ifNull':
              field, fallback = values
              try:
                  out_value = self.parse(field)
                  if out_value is not None:
                      return out_value
              except KeyError:
                  pass
              return self.parse(fallback)
          if operator == '$cond':
              if isinstance(values, list):
                  condition, true_case, false_case = values
              elif isinstance(values, dict):
                  condition = values['if']
                  true_case = values['then']
                  false_case = values['else']
              condition_value = self._parse_to_bool(condition)
              expression = true_case if condition_value else false_case
              return self.parse(expression)
          # This should never happen: it is only a safe fallback if something went wrong.
          raise NotImplementedError(  # pragma: no cover
              "Although '%s' is a valid conditional operator for the "
              'aggregation pipeline, it is currently not implemented '
              ' in Mongomock.' % operator)
  
      def _handle_control_flow_operator(self, operator, values):
          if operator == '$switch':
              if not isinstance(values, dict):
                  raise OperationFailure(
                      '$switch requires an object as an argument, '
                      'found: %s' % type(values)
                  )
  
              branches = values.get('branches', [])
              if not isinstance(branches, (list, tuple)):
                  raise OperationFailure(
                      "$switch expected an array for 'branches', "
                      'found: %s' % type(branches)
                  )
              if not branches:
                  raise OperationFailure(
                      '$switch requires at least one branch.'
                  )
  
              for branch in branches:
                  if not isinstance(branch, dict):
                      raise OperationFailure(
                          '$switch expected each branch to be an object, '
                          'found: %s' % type(branch)
                      )
                  if 'case' not in branch:
                      raise OperationFailure(
                          "$switch requires each branch have a 'case' expression"
                      )
                  if 'then' not in branch:
                      raise OperationFailure(
                          "$switch requires each branch have a 'then' expression."
                      )
  
              for branch in branches:
                  if self._parse_to_bool(branch['case']):
                      return self.parse(branch['then'])
  
              if 'default' not in values:
                  raise OperationFailure(
                      '$switch could not find a matching branch for an input, '
                      'and no default was specified.'
                  )
              return self.parse(values['default'])
  
          # This should never happen: it is only a safe fallback if something went wrong.
          raise NotImplementedError(  # pragma: no cover
              "Although '%s' is a valid control flow operator for the "
              'aggregation pipeline, it is currently not implemented '
              'in Mongomock.' % operator)
  
      def _handle_set_operator(self, operator, values):
          if operator == '$in':
              expression, array = values
              return self.parse(expression) in self.parse(array)
          if operator == '$setUnion':
              result = []
              for set_value in values:
                  for value in self.parse(set_value):
                      if value not in result:
                          result.append(value)
              return result
          if operator == '$setEquals':
              set_values = [set(self.parse(value)) for value in values]
              for set1, set2 in itertools.combinations(set_values, 2):
                  if set1 != set2:
                      return False
              return True
          raise NotImplementedError(
              "Although '%s' is a valid set operator for the aggregation "
              'pipeline, it is currently not implemented in Mongomock.' % operator)
  
  
  def _parse_expression(expression, doc_dict, ignore_missing_keys=False):
      """Parse an expression.
  
      Args:
          expression: an Aggregate Expression, see
              https://docs.mongodb.com/manual/meta/aggregation-quick-reference/#aggregation-expressions.
          doc_dict: the document on which to evaluate the expression.
          ignore_missing_keys: if True, missing keys evaluated by the expression are ignored silently
              if it is possible.
      """
      return _Parser(doc_dict, ignore_missing_keys=ignore_missing_keys).parse(expression)
  
  
  filtering.register_parse_expression(_parse_expression)
  
  
  def _accumulate_group(output_fields, group_list):
      doc_dict = {}
      for field, value in six.iteritems(output_fields):
          if field == '_id':
              continue
          for operator, key in six.iteritems(value):
              values = []
              for doc in group_list:
                  try:
                      values.append(_parse_expression(key, doc))
                  except KeyError:
                      continue
              if operator in _GROUPING_OPERATOR_MAP:
                  doc_dict[field] = _GROUPING_OPERATOR_MAP[operator](values)
              elif operator == '$first':
                  doc_dict[field] = values[0] if values else None
              elif operator == '$last':
                  doc_dict[field] = values[-1] if values else None
              elif operator == '$addToSet':
                  value = []
                  val_it = (val or None for val in values)
                  # Don't use set in case elt in not hashable (like dicts).
                  for elt in val_it:
                      if elt not in value:
                          value.append(elt)
                  doc_dict[field] = value
              elif operator == '$push':
                  if field not in doc_dict:
                      doc_dict[field] = values
                  else:
                      doc_dict[field].extend(values)
              elif operator in group_operators:
                  raise NotImplementedError(
                      'Although %s is a valid group operator for the '
                      'aggregation pipeline, it is currently not implemented '
                      'in Mongomock.' % operator)
              else:
                  raise NotImplementedError(
                      '%s is not a valid group operator for the aggregation '
                      'pipeline. See http://docs.mongodb.org/manual/meta/'
                      'aggregation-quick-reference/ for a complete list of '
                      'valid operators.' % operator)
      return doc_dict
  
  
  def _fix_sort_key(key_getter):
      def fixed_getter(doc):
          key = key_getter(doc)
          # Convert dictionaries to make sorted() work in Python 3.
          if isinstance(key, dict):
              return [(k, v) for (k, v) in sorted(key.items())]
          return key
      return fixed_getter
  
  
  def _handle_lookup_stage(in_collection, database, options):
      for operator in ('let', 'pipeline'):
          if operator in options:
              raise NotImplementedError(
                  "Although '%s' is a valid lookup operator for the "
                  'aggregation pipeline, it is currently not '
                  'implemented in Mongomock.' % operator)
      for operator in ('from', 'localField', 'foreignField', 'as'):
          if operator not in options:
              raise OperationFailure(
                  "Must specify '%s' field for a $lookup" % operator)
          if not isinstance(options[operator], six.string_types):
              raise OperationFailure(
                  'Arguments to $lookup must be strings')
          if operator in ('as', 'localField', 'foreignField') and \
                  options[operator].startswith('$'):
              raise OperationFailure(
                  "FieldPath field names may not start with '$'")
          if operator == 'as' and \
                  '.' in options[operator]:
              raise NotImplementedError(
                  "Although '.' is valid in the 'as' "
                  'parameters for the lookup stage of the aggregation '
                  'pipeline, it is currently not implemented in Mongomock.')
  
      foreign_name = options['from']
      local_field = options['localField']
      foreign_field = options['foreignField']
      local_name = options['as']
      foreign_collection = database.get_collection(foreign_name)
      for doc in in_collection:
          try:
              query = helpers.get_value_by_dot(doc, local_field)
          except KeyError:
              query = None
          if isinstance(query, list):
              query = {'$in': query}
          matches = foreign_collection.find({foreign_field: query})
          doc[local_name] = [foreign_doc for foreign_doc in matches]
  
      return in_collection
  
  
  def _recursive_get(match, nested_fields):
      head = match.get(nested_fields[0])
      remaining_fields = nested_fields[1:]
      if not remaining_fields:
          # Final/last field reached.
          yield head
          return
      # More fields to go, must be list, tuple, or dict.
      if isinstance(head, (list, tuple)):
          for m in head:
              # Yield from _recursive_get(m, remaining_fields).
              for answer in _recursive_get(m, remaining_fields):
                  yield answer
      elif isinstance(head, dict):
          # Yield from _recursive_get(head, remaining_fields).
          for answer in _recursive_get(head, remaining_fields):
              yield answer
  
  
  def _handle_graph_lookup_stage(in_collection, database, options):
      if not isinstance(options.get('maxDepth', 0), six.integer_types):
          raise OperationFailure(
              "Argument 'maxDepth' to $graphLookup must be a number")
      if not isinstance(options.get('restrictSearchWithMatch', {}), dict):
          raise OperationFailure(
              "Argument 'restrictSearchWithMatch' to $graphLookup must be a Dictionary")
      if not isinstance(options.get('depthField', ''), six.string_types):
          raise OperationFailure(
              "Argument 'depthField' to $graphlookup must be a string")
      if 'startWith' not in options:
          raise OperationFailure(
              "Must specify 'startWith' field for a $graphLookup")
      for operator in ('as', 'connectFromField', 'connectToField', 'from'):
          if operator not in options:
              raise OperationFailure(
                  "Must specify '%s' field for a $graphLookup" % operator)
          if not isinstance(options[operator], six.string_types):
              raise OperationFailure(
                  "Argument '%s' to $graphLookup must be string" % operator)
          if options[operator].startswith('$'):
              raise OperationFailure("FieldPath field names may not start with '$'")
          if operator == 'as' and '.' in options[operator]:
              raise NotImplementedError(
                  "Although '.' is valid in the '%s' "
                  'parameter for the $graphLookup stage of the aggregation '
                  'pipeline, it is currently not implemented in Mongomock.' % operator)
  
      foreign_name = options['from']
      start_with = options['startWith']
      connect_from_field = options['connectFromField']
      connect_to_field = options['connectToField']
      local_name = options['as']
      max_depth = options.get('maxDepth', None)
      depth_field = options.get('depthField', None)
      restrict_search_with_match = options.get('restrictSearchWithMatch', {})
      foreign_collection = database.get_collection(foreign_name)
      out_doc = copy.deepcopy(in_collection)  # TODO(pascal): speed the deep copy
  
      def _find_matches_for_depth(query):
          if isinstance(query, list):
              query = {'$in': query}
          matches = foreign_collection.find({connect_to_field: query})
          new_matches = []
          for new_match in matches:
              if filtering.filter_applies(restrict_search_with_match, new_match) \
                      and new_match['_id'] not in found_items:
                  if depth_field is not None:
                      new_match = collections.OrderedDict(new_match, **{depth_field: depth})
                  new_matches.append(new_match)
                  found_items.add(new_match['_id'])
          return new_matches
  
      for doc in out_doc:
          found_items = set()
          depth = 0
          try:
              result = _parse_expression(start_with, doc)
          except KeyError:
              continue
          origin_matches = doc[local_name] = _find_matches_for_depth(result)
          while origin_matches and (max_depth is None or depth < max_depth):
              depth += 1
              newly_discovered_matches = []
              for match in origin_matches:
                  nested_fields = connect_from_field.split('.')
                  for match_target in _recursive_get(match, nested_fields):
                      newly_discovered_matches += _find_matches_for_depth(match_target)
              doc[local_name] += newly_discovered_matches
              origin_matches = newly_discovered_matches
      return out_doc
  
  
  def _handle_group_stage(in_collection, unused_database, options):
      grouped_collection = []
      _id = options['_id']
      if _id:
  
          def _key_getter(doc):
              try:
                  return _parse_expression(_id, doc)
              except KeyError:
                  return None
  
          def _sort_key_getter(doc):
              return filtering.BsonComparable(_key_getter(doc))
  
          # Sort the collection only for the itertools.groupby.
          # $group does not order its output document.
          sorted_collection = sorted(in_collection, key=_sort_key_getter)
          grouped = itertools.groupby(sorted_collection, _key_getter)
      else:
          grouped = [(None, in_collection)]
  
      for doc_id, group in grouped:
          group_list = ([x for x in group])
          doc_dict = _accumulate_group(options, group_list)
          doc_dict['_id'] = doc_id
          grouped_collection.append(doc_dict)
  
      return grouped_collection
  
  
  def _handle_bucket_stage(in_collection, unused_database, options):
      unknown_options = set(options) - {'groupBy', 'boundaries', 'output', 'default'}
      if unknown_options:
          raise OperationFailure(
              'Unrecognized option to $bucket: %s.' % unknown_options.pop())
      if 'groupBy' not in options or 'boundaries' not in options:
          raise OperationFailure(
              "$bucket requires 'groupBy' and 'boundaries' to be specified.")
      group_by = options['groupBy']
      boundaries = options['boundaries']
      if not isinstance(boundaries, list):
          raise OperationFailure(
              "The $bucket 'boundaries' field must be an array, but found type: %s"
              % type(boundaries))
      if len(boundaries) < 2:
          raise OperationFailure(
              "The $bucket 'boundaries' field must have at least 2 values, but "
              'found %d value(s).' % len(boundaries))
      if sorted(boundaries) != boundaries:
          raise OperationFailure(
              "The 'boundaries' option to $bucket must be sorted in ascending order")
      output_fields = options.get('output', {'count': {'$sum': 1}})
      default_value = options.get('default', None)
      try:
          is_default_last = default_value >= boundaries[-1]
      except TypeError:
          is_default_last = True
  
      def _get_default_bucket():
          try:
              return options['default']
          except KeyError as err:
              raise_from(OperationFailure(
                  '$bucket could not find a matching branch for '
                  'an input, and no default was specified.'), err)
  
      def _get_bucket_id(doc):
          """Get the bucket ID for a document.
  
          Note that it actually returns a tuple with the first
          param being a sort key to sort the default bucket even
          if it's not the same type as the boundaries.
          """
          try:
              value = _parse_expression(group_by, doc)
          except KeyError:
              return (is_default_last, _get_default_bucket())
          index = bisect.bisect_right(boundaries, value)
          if index and index < len(boundaries):
              return (False, boundaries[index - 1])
          return (is_default_last, _get_default_bucket())
  
      in_collection = ((_get_bucket_id(doc), doc) for doc in in_collection)
      out_collection = sorted(in_collection, key=lambda kv: kv[0])
      grouped = itertools.groupby(out_collection, lambda kv: kv[0])
  
      out_collection = []
      for (unused_key, doc_id), group in grouped:
          group_list = [kv[1] for kv in group]
          doc_dict = _accumulate_group(output_fields, group_list)
          doc_dict['_id'] = doc_id
          out_collection.append(doc_dict)
      return out_collection
  
  
  def _handle_sample_stage(in_collection, unused_database, options):
      if not isinstance(options, dict):
          raise OperationFailure('the $sample stage specification must be an object')
      size = options.pop('size', None)
      if size is None:
          raise OperationFailure('$sample stage must specify a size')
      if options:
          raise OperationFailure('unrecognized option to $sample: %s' % set(options).pop())
      shuffled = list(in_collection)
      _random.shuffle(shuffled)
      return shuffled[:size]
  
  
  def _handle_sort_stage(in_collection, unused_database, options):
      sort_array = reversed([{x: y} for x, y in options.items()])
      sorted_collection = in_collection
      for sort_pair in sort_array:
          for sortKey, sortDirection in sort_pair.items():
              sorted_collection = sorted(
                  sorted_collection,
                  key=lambda x: filtering.resolve_sort_key(sortKey, x),
                  reverse=sortDirection < 0)
      return sorted_collection
  
  
  def _handle_unwind_stage(in_collection, unused_database, options):
      if not isinstance(options, dict):
          options = {'path': options}
      path = options['path']
      if not isinstance(path, six.string_types) or path[0] != '$':
          raise ValueError(
              '$unwind failed: exception: field path references must be prefixed '
              "with a '$' '%s'" % path)
      path = path[1:]
      should_preserve_null_and_empty = options.get('preserveNullAndEmptyArrays')
      include_array_index = options.get('includeArrayIndex')
      unwound_collection = []
      for doc in in_collection:
          try:
              array_value = helpers.get_value_by_dot(doc, path)
          except KeyError:
              if should_preserve_null_and_empty:
                  unwound_collection.append(doc)
              continue
          if array_value is None:
              if should_preserve_null_and_empty:
                  unwound_collection.append(doc)
              continue
          if array_value == []:
              if should_preserve_null_and_empty:
                  new_doc = copy.deepcopy(doc)
                  # We just ran a get_value_by_dot so we know the value exists.
                  helpers.delete_value_by_dot(new_doc, path)
                  unwound_collection.append(new_doc)
              continue
          if isinstance(array_value, list):
              iter_array = enumerate(array_value)
          else:
              iter_array = [(None, array_value)]
          for index, field_item in iter_array:
              new_doc = copy.deepcopy(doc)
              new_doc = helpers.set_value_by_dot(new_doc, path, field_item)
              if include_array_index:
                  new_doc = helpers.set_value_by_dot(new_doc, include_array_index, index)
              unwound_collection.append(new_doc)
  
      return unwound_collection
  
  
  # TODO(pascal): Combine with the equivalent function in collection but check
  # what are the allowed overriding.
  def _combine_projection_spec(filter_list, original_filter, prefix=''):
      """Re-format a projection fields spec into a nested dictionary.
  
      e.g: ['a', 'b.c', 'b.d'] => {'a': 1, 'b': {'c': 1, 'd': 1}}
      """
      if not isinstance(filter_list, list):
          return filter_list
  
      filter_dict = collections.OrderedDict()
  
      for key in filter_list:
          field, separator, subkey = key.partition('.')
          if not separator:
              if isinstance(filter_dict.get(field), list):
                  other_key = field + '.' + filter_dict[field][0]
                  raise OperationFailure(
                      'Invalid $project :: caused by :: specification contains two conflicting paths.'
                      ' Cannot specify both %s and %s: %s' % (
                          repr(prefix + field), repr(prefix + other_key), original_filter))
              filter_dict[field] = 1
              continue
          if not isinstance(filter_dict.get(field, []), list):
              raise OperationFailure(
                  'Invalid $project :: caused by :: specification contains two conflicting paths.'
                  ' Cannot specify both %s and %s: %s' % (
                      repr(prefix + field), repr(prefix + key), original_filter))
          filter_dict[field] = filter_dict.get(field, []) + [subkey]
  
      return collections.OrderedDict(
          (k, _combine_projection_spec(v, original_filter, prefix='%s%s.' % (prefix, k)))
          for k, v in six.iteritems(filter_dict)
      )
  
  
  def _project_by_spec(doc, proj_spec, is_include):
      output = {}
      for key, value in six.iteritems(doc):
          if key not in proj_spec:
              if not is_include:
                  output[key] = value
              continue
  
          if not isinstance(proj_spec[key], dict):
              if is_include:
                  output[key] = value
              continue
  
          if isinstance(value, dict):
              output[key] = _project_by_spec(value, proj_spec[key], is_include)
          elif isinstance(value, list):
              output[key] = [_project_by_spec(array_value, proj_spec[key], is_include)
                             for array_value in value if isinstance(array_value, dict)]
          elif not is_include:
              output[key] = value
  
      return output
  
  
  def _handle_replace_root_stage(in_collection, unused_database, options):
      if 'newRoot' not in options:
          raise OperationFailure("Parameter 'newRoot' is missing for $replaceRoot operation.")
      new_root = options['newRoot']
      out_collection = []
      for doc in in_collection:
          try:
              new_doc = _parse_expression(new_root, doc, ignore_missing_keys=True)
          except KeyError:
              new_doc = NOTHING
          if not isinstance(new_doc, dict):
              raise OperationFailure(
                  "'newRoot' expression must evaluate to an object, but resulting value was: {}"
                  .format(new_doc))
          out_collection.append(new_doc)
      return out_collection
  
  
  def _handle_project_stage(in_collection, unused_database, options):
      filter_list = []
      method = None
      include_id = options.get('_id')
      # Compute new values for each field, except inclusion/exclusions that are
      # handled in one final step.
      new_fields_collection = None
      for field, value in six.iteritems(options):
          if method is None and (field != '_id' or value):
              method = 'include' if value else 'exclude'
          elif method == 'include' and not value and field != '_id':
              raise OperationFailure(
                  'Bad projection specification, cannot exclude fields '
                  "other than '_id' in an inclusion projection: %s" % options)
          elif method == 'exclude' and value:
              raise OperationFailure(
                  'Bad projection specification, cannot include fields '
                  'or add computed fields during an exclusion projection: %s' % options)
          if value in (0, 1, True, False):
              if field != '_id':
                  filter_list.append(field)
              continue
          if not new_fields_collection:
              new_fields_collection = [{} for unused_doc in in_collection]
  
          for in_doc, out_doc in zip(in_collection, new_fields_collection):
              try:
                  out_doc[field] = _parse_expression(value, in_doc, ignore_missing_keys=True)
              except KeyError:
                  # Ignore missing key.
                  pass
      if (method == 'include') == (include_id is not False and include_id != 0):
          filter_list.append('_id')
  
      if not filter_list:
          return new_fields_collection
  
      # Final steps: include or exclude fields and merge with newly created fields.
      projection_spec = _combine_projection_spec(filter_list, original_filter=options)
      out_collection = [
          _project_by_spec(doc, projection_spec, is_include=(method == 'include'))
          for doc in in_collection
      ]
      if new_fields_collection:
          return [
              dict(a, **b)
              for a, b in zip(out_collection, new_fields_collection)
          ]
      return out_collection
  
  
  def _handle_add_fields_stage(in_collection, unused_database, options):
      if not options:
          raise OperationFailure(
              'Invalid $addFields :: caused by :: specification must have at least one field')
      out_collection = [dict(doc) for doc in in_collection]
      for field, value in six.iteritems(options):
          for in_doc, out_doc in zip(in_collection, out_collection):
              try:
                  out_value = _parse_expression(value, in_doc, ignore_missing_keys=True)
              except KeyError:
                  continue
              parts = field.split('.')
              for subfield in parts[:-1]:
                  out_doc[subfield] = out_doc.get(subfield, {})
                  if not isinstance(out_doc[subfield], dict):
                      out_doc[subfield] = {}
                  out_doc = out_doc[subfield]
              out_doc[parts[-1]] = out_value
      return out_collection
  
  
  def _handle_out_stage(in_collection, database, options):
      # TODO(MetrodataTeam): should leave the origin collection unchanged
      out_collection = database.get_collection(options)
      if out_collection.count() > 0:
          out_collection.drop()
      if in_collection:
          out_collection.insert_many(in_collection)
      return in_collection
  
  
  def _handle_count_stage(in_collection, database, options):
      if not isinstance(options, str) or options == '':
          raise OperationFailure('the count field must be a non-empty string')
      elif options.startswith('$'):
          raise OperationFailure('the count field cannot be a $-prefixed path')
      elif '.' in options:
          raise OperationFailure("the count field cannot contain '.'")
      return [{options: len(in_collection)}]
  
  
  def _handle_facet_stage(in_collection, database, options):
      out_collection_by_pipeline = {}
      for pipeline_title, pipeline in options.items():
          out_collection_by_pipeline[pipeline_title] = list(process_pipeline(
              in_collection, database, pipeline, None))
      return [out_collection_by_pipeline]
  
  
  def _handle_match_stage(in_collection, database, options):
      spec = helpers.patch_datetime_awareness_in_document(options)
      return [
          doc for doc in in_collection
          if filtering.filter_applies(spec, helpers.patch_datetime_awareness_in_document(doc))
      ]
  
  
  _PIPELINE_HANDLERS = {
      '$addFields': _handle_add_fields_stage,
      '$bucket': _handle_bucket_stage,
      '$bucketAuto': None,
      '$collStats': None,
      '$count': _handle_count_stage,
      '$currentOp': None,
      '$facet': _handle_facet_stage,
      '$geoNear': None,
      '$graphLookup': _handle_graph_lookup_stage,
      '$group': _handle_group_stage,
      '$indexStats': None,
      '$limit': lambda c, d, o: c[:o],
      '$listLocalSessions': None,
      '$listSessions': None,
      '$lookup': _handle_lookup_stage,
      '$match': _handle_match_stage,
      '$merge': None,
      '$out': _handle_out_stage,
      '$planCacheStats': None,
      '$project': _handle_project_stage,
      '$redact': None,
      '$replaceRoot': _handle_replace_root_stage,
      '$replaceWith': None,
      '$sample': _handle_sample_stage,
      '$set': _handle_add_fields_stage,
      '$skip': lambda c, d, o: c[o:],
      '$sort': _handle_sort_stage,
      '$sortByCount': None,
      '$unset': None,
      '$unwind': _handle_unwind_stage,
  }
  
  
  def process_pipeline(collection, database, pipeline, session):
      if session:
          raise NotImplementedError('Mongomock does not handle sessions yet')
  
      for stage in pipeline:
          for operator, options in six.iteritems(stage):
              try:
                  handler = _PIPELINE_HANDLERS[operator]
              except KeyError as err:
                  raise_from(NotImplementedError(
                      '%s is not a valid operator for the aggregation pipeline. '
                      'See http://docs.mongodb.org/manual/meta/aggregation-quick-reference/ '
                      'for a complete list of valid operators.' % operator), err)
              if not handler:
                  raise NotImplementedError(
                      "Although '%s' is a valid operator for the aggregation pipeline, it is "
                      'currently not implemented in Mongomock.' % operator)
              collection = handler(collection, database, options)
  
      return command_cursor.CommandCursor(collection)
  