text: import bisect
  import copy
  import inspect
  from collections import defaultdict
  
  from django.apps import apps
  from django.conf import settings
  from django.core.exceptions import FieldDoesNotExist, ImproperlyConfigured
  from django.db import connections
  from django.db.models import AutoField, Manager, OrderWrt, UniqueConstraint
  from django.db.models.query_utils import PathInfo
  from django.utils.datastructures import ImmutableList, OrderedSet
  from django.utils.functional import cached_property
  from django.utils.module_loading import import_string
  from django.utils.text import camel_case_to_spaces, format_lazy
  from django.utils.translation import override
  
  PROXY_PARENTS = object()
  
  EMPTY_RELATION_TREE = ()
  
  IMMUTABLE_WARNING = (
      "The return type of '%s' should never be mutated. If you want to manipulate this list "
      "for your own use, make a copy first."
  )
  
  DEFAULT_NAMES = (
      'verbose_name', 'verbose_name_plural', 'db_table', 'ordering',
      'unique_together', 'permissions', 'get_latest_by', 'order_with_respect_to',
      'app_label', 'db_tablespace', 'abstract', 'managed', 'proxy', 'swappable',
      'auto_created', 'index_together', 'apps', 'default_permissions',
      'select_on_save', 'default_related_name', 'required_db_features',
      'required_db_vendor', 'base_manager_name', 'default_manager_name',
      'indexes', 'constraints',
  )
  
  
  def normalize_together(option_together):
      """
      option_together can be either a tuple of tuples, or a single
      tuple of two strings. Normalize it to a tuple of tuples, so that
      calling code can uniformly expect that.
      """
      try:
          if not option_together:
              return ()
          if not isinstance(option_together, (tuple, list)):
              raise TypeError
          first_element = option_together[0]
          if not isinstance(first_element, (tuple, list)):
              option_together = (option_together,)
          # Normalize everything to tuples
          return tuple(tuple(ot) for ot in option_together)
      except TypeError:
          # If the value of option_together isn't valid, return it
          # verbatim; this will be picked up by the check framework later.
          return option_together
  
  
  def make_immutable_fields_list(name, data):
      return ImmutableList(data, warning=IMMUTABLE_WARNING % name)
  
  
  class Options:
      FORWARD_PROPERTIES = {
          'fields', 'many_to_many', 'concrete_fields', 'local_concrete_fields',
          '_forward_fields_map', 'managers', 'managers_map', 'base_manager',
          'default_manager',
      }
      REVERSE_PROPERTIES = {'related_objects', 'fields_map', '_relation_tree'}
  
      default_apps = apps
  
      def __init__(self, meta, app_label=None):
          self._get_fields_cache = {}
          self.local_fields = []
          self.local_many_to_many = []
          self.private_fields = []
          self.local_managers = []
          self.base_manager_name = None
          self.default_manager_name = None
          self.model_name = None
          self.verbose_name = None
          self.verbose_name_plural = None
          self.db_table = ''
          self.ordering = []
          self._ordering_clash = False
          self.indexes = []
          self.constraints = []
          self.unique_together = []
          self.index_together = []
          self.select_on_save = False
          self.default_permissions = ('add', 'change', 'delete', 'view')
          self.permissions = []
          self.object_name = None
          self.app_label = app_label
          self.get_latest_by = None
          self.order_with_respect_to = None
          self.db_tablespace = settings.DEFAULT_TABLESPACE
          self.required_db_features = []
          self.required_db_vendor = None
          self.meta = meta
          self.pk = None
          self.auto_field = None
          self.abstract = False
          self.managed = True
          self.proxy = False
          # For any class that is a proxy (including automatically created
          # classes for deferred object loading), proxy_for_model tells us
          # which class this model is proxying. Note that proxy_for_model
          # can create a chain of proxy models. For non-proxy models, the
          # variable is always None.
          self.proxy_for_model = None
          # For any non-abstract class, the concrete class is the model
          # in the end of the proxy_for_model chain. In particular, for
          # concrete models, the concrete_model is always the class itself.
          self.concrete_model = None
          self.swappable = None
          self.parents = {}
          self.auto_created = False
  
          # List of all lookups defined in ForeignKey 'limit_choices_to' options
          # from *other* models. Needed for some admin checks. Internal use only.
          self.related_fkey_lookups = []
  
          # A custom app registry to use, if you're making a separate model set.
          self.apps = self.default_apps
  
          self.default_related_name = None
  
      @property
      def label(self):
          return '%s.%s' % (self.app_label, self.object_name)
  
      @property
      def label_lower(self):
          return '%s.%s' % (self.app_label, self.model_name)
  
      @property
      def app_config(self):
          # Don't go through get_app_config to avoid triggering imports.
          return self.apps.app_configs.get(self.app_label)
  
      @property
      def installed(self):
          return self.app_config is not None
  
      def contribute_to_class(self, cls, name):
          from django.db import connection
          from django.db.backends.utils import truncate_name
  
          cls._meta = self
          self.model = cls
          # First, construct the default values for these options.
          self.object_name = cls.__name__
          self.model_name = self.object_name.lower()
          self.verbose_name = camel_case_to_spaces(self.object_name)
  
          # Store the original user-defined values for each option,
          # for use when serializing the model definition
          self.original_attrs = {}
  
          # Next, apply any overridden values from 'class Meta'.
          if self.meta:
              meta_attrs = self.meta.__dict__.copy()
              for name in self.meta.__dict__:
                  # Ignore any private attributes that Django doesn't care about.
                  # NOTE: We can't modify a dictionary's contents while looping
                  # over it, so we loop over the *original* dictionary instead.
                  if name.startswith('_'):
                      del meta_attrs[name]
              for attr_name in DEFAULT_NAMES:
                  if attr_name in meta_attrs:
                      setattr(self, attr_name, meta_attrs.pop(attr_name))
                      self.original_attrs[attr_name] = getattr(self, attr_name)
                  elif hasattr(self.meta, attr_name):
                      setattr(self, attr_name, getattr(self.meta, attr_name))
                      self.original_attrs[attr_name] = getattr(self, attr_name)
  
              self.unique_together = normalize_together(self.unique_together)
              self.index_together = normalize_together(self.index_together)
              # App label/class name interpolation for names of constraints and
              # indexes.
              if not getattr(cls._meta, 'abstract', False):
                  for attr_name in {'constraints', 'indexes'}:
                      objs = getattr(self, attr_name, [])
                      setattr(self, attr_name, self._format_names_with_class(cls, objs))
  
              # verbose_name_plural is a special case because it uses a 's'
              # by default.
              if self.verbose_name_plural is None:
                  self.verbose_name_plural = format_lazy('{}s', self.verbose_name)
  
              # order_with_respect_and ordering are mutually exclusive.
              self._ordering_clash = bool(self.ordering and self.order_with_respect_to)
  
              # Any leftover attributes must be invalid.
              if meta_attrs != {}:
                  raise TypeError("'class Meta' got invalid attribute(s): %s" % ','.join(meta_attrs))
          else:
              self.verbose_name_plural = format_lazy('{}s', self.verbose_name)
          del self.meta
  
          # If the db_table wasn't provided, use the app_label + model_name.
          if not self.db_table:
              self.db_table = "%s_%s" % (self.app_label, self.model_name)
              self.db_table = truncate_name(self.db_table, connection.ops.max_name_length())
  
      def _format_names_with_class(self, cls, objs):
          """App label/class name interpolation for object names."""
          new_objs = []
          for obj in objs:
              obj = obj.clone()
              obj.name = obj.name % {
                  'app_label': cls._meta.app_label.lower(),
                  'class': cls.__name__.lower(),
              }
              new_objs.append(obj)
          return new_objs
  
      def _get_default_pk_class(self):
          pk_class_path = getattr(
              self.app_config,
              'default_auto_field',
              settings.DEFAULT_AUTO_FIELD,
          )
          if self.app_config and self.app_config._is_default_auto_field_overridden:
              app_config_class = type(self.app_config)
              source = (
                  f'{app_config_class.__module__}.'
                  f'{app_config_class.__qualname__}.default_auto_field'
              )
          else:
              source = 'DEFAULT_AUTO_FIELD'
          if not pk_class_path:
              raise ImproperlyConfigured(f'{source} must not be empty.')
          try:
              pk_class = import_string(pk_class_path)
          except ImportError as e:
              msg = (
                  f"{source} refers to the module '{pk_class_path}' that could "
                  f"not be imported."
              )
              raise ImproperlyConfigured(msg) from e
          if not issubclass(pk_class, AutoField):
              raise ValueError(
                  f"Primary key '{pk_class_path}' referred by {source} must "
                  f"subclass AutoField."
              )
          return pk_class
  
      def _prepare(self, model):
          if self.order_with_respect_to:
              # The app registry will not be ready at this point, so we cannot
              # use get_field().
              query = self.order_with_respect_to
              try:
                  self.order_with_respect_to = next(
                      f for f in self._get_fields(reverse=False)
                      if f.name == query or f.attname == query
                  )
              except StopIteration:
                  raise FieldDoesNotExist("%s has no field named '%s'" % (self.object_name, query))
  
              self.ordering = ('_order',)
              if not any(isinstance(field, OrderWrt) for field in model._meta.local_fields):
                  model.add_to_class('_order', OrderWrt())
          else:
              self.order_with_respect_to = None
  
          if self.pk is None:
              if self.parents:
                  # Promote the first parent link in lieu of adding yet another
                  # field.
                  field = next(iter(self.parents.values()))
                  # Look for a local field with the same name as the
                  # first parent link. If a local field has already been
                  # created, use it instead of promoting the parent
                  already_created = [fld for fld in self.local_fields if fld.name == field.name]
                  if already_created:
                      field = already_created[0]
                  field.primary_key = True
                  self.setup_pk(field)
              else:
                  pk_class = self._get_default_pk_class()
                  auto = pk_class(verbose_name='ID', primary_key=True, auto_created=True)
                  model.add_to_class('id', auto)
  
      def add_manager(self, manager):
          self.local_managers.append(manager)
          self._expire_cache()
  
      def add_field(self, field, private=False):
          # Insert the given field in the order in which it was created, using
          # the "creation_counter" attribute of the field.
          # Move many-to-many related fields from self.fields into
          # self.many_to_many.
          if private:
              self.private_fields.append(field)
          elif field.is_relation and field.many_to_many:
              bisect.insort(self.local_many_to_many, field)
          else:
              bisect.insort(self.local_fields, field)
              self.setup_pk(field)
  
          # If the field being added is a relation to another known field,
          # expire the cache on this field and the forward cache on the field
          # being referenced, because there will be new relationships in the
          # cache. Otherwise, expire the cache of references *to* this field.
          # The mechanism for getting at the related model is slightly odd -
          # ideally, we'd just ask for field.related_model. However, related_model
          # is a cached property, and all the models haven't been loaded yet, so
          # we need to make sure we don't cache a string reference.
          if field.is_relation and hasattr(field.remote_field, 'model') and field.remote_field.model:
              try:
                  field.remote_field.model._meta._expire_cache(forward=False)
              except AttributeError:
                  pass
              self._expire_cache()
          else:
              self._expire_cache(reverse=False)
  
      def setup_pk(self, field):
          if not self.pk and field.primary_key:
              self.pk = field
              field.serialize = False
  
      def setup_proxy(self, target):
          """
          Do the internal setup so that the current model is a proxy for
          "target".
          """
          self.pk = target._meta.pk
          self.proxy_for_model = target
          self.db_table = target._meta.db_table
  
      def __repr__(self):
          return '<Options for %s>' % self.object_name
  
      def __str__(self):
          return self.label_lower
  
      def can_migrate(self, connection):
          """
          Return True if the model can/should be migrated on the `connection`.
          `connection` can be either a real connection or a connection alias.
          """
          if self.proxy or self.swapped or not self.managed:
              return False
          if isinstance(connection, str):
              connection = connections[connection]
          if self.required_db_vendor:
              return self.required_db_vendor == connection.vendor
          if self.required_db_features:
              return all(getattr(connection.features, feat, False)
                         for feat in self.required_db_features)
          return True
  
      @property
      def verbose_name_raw(self):
          """Return the untranslated verbose name."""
          with override(None):
              return str(self.verbose_name)
  
      @property
      def swapped(self):
          """
          Has this model been swapped out for another? If so, return the model
          name of the replacement; otherwise, return None.
  
          For historical reasons, model name lookups using get_model() are
          case insensitive, so we make sure we are case insensitive here.
          """
          if self.swappable:
              swapped_for = getattr(settings, self.swappable, None)
              if swapped_for:
                  try:
                      swapped_label, swapped_object = swapped_for.split('.')
                  except ValueError:
                      # setting not in the format app_label.model_name
                      # raising ImproperlyConfigured here causes problems with
                      # test cleanup code - instead it is raised in get_user_model
                      # or as part of validation.
                      return swapped_for
  
                  if '%s.%s' % (swapped_label, swapped_object.lower()) != self.label_lower:
                      return swapped_for
          return None
  
      @cached_property
      def managers(self):
          managers = []
          seen_managers = set()
          bases = (b for b in self.model.mro() if hasattr(b, '_meta'))
          for depth, base in enumerate(bases):
              for manager in base._meta.local_managers:
                  if manager.name in seen_managers:
                      continue
  
                  manager = copy.copy(manager)
                  manager.model = self.model
                  seen_managers.add(manager.name)
                  managers.append((depth, manager.creation_counter, manager))
  
          return make_immutable_fields_list(
              "managers",
              (m[2] for m in sorted(managers)),
          )
  
      @cached_property
      def managers_map(self):
          return {manager.name: manager for manager in self.managers}
  
      @cached_property
      def base_manager(self):
          base_manager_name = self.base_manager_name
          if not base_manager_name:
              # Get the first parent's base_manager_name if there's one.
              for parent in self.model.mro()[1:]:
                  if hasattr(parent, '_meta'):
                      if parent._base_manager.name != '_base_manager':
                          base_manager_name = parent._base_manager.name
                      break
  
          if base_manager_name:
              try:
                  return self.managers_map[base_manager_name]
              except KeyError:
                  raise ValueError(
                      "%s has no manager named %r" % (
                          self.object_name,
                          base_manager_name,
                      )
                  )
  
          manager = Manager()
          manager.name = '_base_manager'
          manager.model = self.model
          manager.auto_created = True
          return manager
  
      @cached_property
      def default_manager(self):
          default_manager_name = self.default_manager_name
          if not default_manager_name and not self.local_managers:
              # Get the first parent's default_manager_name if there's one.
              for parent in self.model.mro()[1:]:
                  if hasattr(parent, '_meta'):
                      default_manager_name = parent._meta.default_manager_name
                      break
  
          if default_manager_name:
              try:
                  return self.managers_map[default_manager_name]
              except KeyError:
                  raise ValueError(
                      "%s has no manager named %r" % (
                          self.object_name,
                          default_manager_name,
                      )
                  )
  
          if self.managers:
              return self.managers[0]
  
      @cached_property
      def fields(self):
          """
          Return a list of all forward fields on the model and its parents,
          excluding ManyToManyFields.
  
          Private API intended only to be used by Django itself; get_fields()
          combined with filtering of field properties is the public API for
          obtaining this field list.
          """
          # For legacy reasons, the fields property should only contain forward
          # fields that are not private or with a m2m cardinality. Therefore we
          # pass these three filters as filters to the generator.
          # The third lambda is a longwinded way of checking f.related_model - we don't
          # use that property directly because related_model is a cached property,
          # and all the models may not have been loaded yet; we don't want to cache
          # the string reference to the related_model.
          def is_not_an_m2m_field(f):
              return not (f.is_relation and f.many_to_many)
  
          def is_not_a_generic_relation(f):
              return not (f.is_relation and f.one_to_many)
  
          def is_not_a_generic_foreign_key(f):
              return not (
                  f.is_relation and f.many_to_one and not (hasattr(f.remote_field, 'model') and f.remote_field.model)
              )
  
          return make_immutable_fields_list(
              "fields",
              (f for f in self._get_fields(reverse=False)
               if is_not_an_m2m_field(f) and is_not_a_generic_relation(f) and is_not_a_generic_foreign_key(f))
          )
  
      @cached_property
      def concrete_fields(self):
          """
          Return a list of all concrete fields on the model and its parents.
  
          Private API intended only to be used by Django itself; get_fields()
          combined with filtering of field properties is the public API for
          obtaining this field list.
          """
          return make_immutable_fields_list(
              "concrete_fields", (f for f in self.fields if f.concrete)
          )
  
      @cached_property
      def local_concrete_fields(self):
          """
          Return a list of all concrete fields on the model.
  
          Private API intended only to be used by Django itself; get_fields()
          combined with filtering of field properties is the public API for
          obtaining this field list.
          """
          return make_immutable_fields_list(
              "local_concrete_fields", (f for f in self.local_fields if f.concrete)
          )
  
      @cached_property
      def many_to_many(self):
          """
          Return a list of all many to many fields on the model and its parents.
  
          Private API intended only to be used by Django itself; get_fields()
          combined with filtering of field properties is the public API for
          obtaining this list.
          """
          return make_immutable_fields_list(
              "many_to_many",
              (f for f in self._get_fields(reverse=False) if f.is_relation and f.many_to_many)
          )
  
      @cached_property
      def related_objects(self):
          """
          Return all related objects pointing to the current model. The related
          objects can come from a one-to-one, one-to-many, or many-to-many field
          relation type.
  
          Private API intended only to be used by Django itself; get_fields()
          combined with filtering of field properties is the public API for
          obtaining this field list.
          """
          all_related_fields = self._get_fields(forward=False, reverse=True, include_hidden=True)
          return make_immutable_fields_list(
              "related_objects",
              (obj for obj in all_related_fields if not obj.hidden or obj.field.many_to_many)
          )
  
      @cached_property
      def _forward_fields_map(self):
          res = {}
          fields = self._get_fields(reverse=False)
          for field in fields:
              res[field.name] = field
              # Due to the way Django's internals work, get_field() should also
              # be able to fetch a field by attname. In the case of a concrete
              # field with relation, includes the *_id name too
              try:
                  res[field.attname] = field
              except AttributeError:
                  pass
          return res
  
      @cached_property
      def fields_map(self):
          res = {}
          fields = self._get_fields(forward=False, include_hidden=True)
          for field in fields:
              res[field.name] = field
              # Due to the way Django's internals work, get_field() should also
              # be able to fetch a field by attname. In the case of a concrete
              # field with relation, includes the *_id name too
              try:
                  res[field.attname] = field
              except AttributeError:
                  pass
          return res
  
      def get_field(self, field_name):
          """
          Return a field instance given the name of a forward or reverse field.
          """
          try:
              # In order to avoid premature loading of the relation tree
              # (expensive) we prefer checking if the field is a forward field.
              return self._forward_fields_map[field_name]
          except KeyError:
              # If the app registry is not ready, reverse fields are
              # unavailable, therefore we throw a FieldDoesNotExist exception.
              if not self.apps.models_ready:
                  raise FieldDoesNotExist(
                      "%s has no field named '%s'. The app cache isn't ready yet, "
                      "so if this is an auto-created related field, it won't "
                      "be available yet." % (self.object_name, field_name)
                  )
  
          try:
              # Retrieve field instance by name from cached or just-computed
              # field map.
              return self.fields_map[field_name]
          except KeyError:
              raise FieldDoesNotExist("%s has no field named '%s'" % (self.object_name, field_name))
  
      def get_base_chain(self, model):
          """
          Return a list of parent classes leading to `model` (ordered from
          closest to most distant ancestor). This has to handle the case where
          `model` is a grandparent or even more distant relation.
          """
          if not self.parents:
              return []
          if model in self.parents:
              return [model]
          for parent in self.parents:
              res = parent._meta.get_base_chain(model)
              if res:
                  res.insert(0, parent)
                  return res
          return []
  
      def get_parent_list(self):
          """
          Return all the ancestors of this model as a list ordered by MRO.
          Useful for determining if something is an ancestor, regardless of lineage.
          """
          result = OrderedSet(self.parents)
          for parent in self.parents:
              for ancestor in parent._meta.get_parent_list():
                  result.add(ancestor)
          return list(result)
  
      def get_ancestor_link(self, ancestor):
          """
          Return the field on the current model which points to the given
          "ancestor". This is possible an indirect link (a pointer to a parent
          model, which points, eventually, to the ancestor). Used when
          constructing table joins for model inheritance.
  
          Return None if the model isn't an ancestor of this one.
          """
          if ancestor in self.parents:
              return self.parents[ancestor]
          for parent in self.parents:
              # Tries to get a link field from the immediate parent
              parent_link = parent._meta.get_ancestor_link(ancestor)
              if parent_link:
                  # In case of a proxied model, the first link
                  # of the chain to the ancestor is that parent
                  # links
                  return self.parents[parent] or parent_link
  
      def get_path_to_parent(self, parent):
          """
          Return a list of PathInfos containing the path from the current
          model to the parent model, or an empty list if parent is not a
          parent of the current model.
          """
          if self.model is parent:
              return []
          # Skip the chain of proxy to the concrete proxied model.
          proxied_model = self.concrete_model
          path = []
          opts = self
          for int_model in self.get_base_chain(parent):
              if int_model is proxied_model:
                  opts = int_model._meta
              else:
                  final_field = opts.parents[int_model]
                  targets = (final_field.remote_field.get_related_field(),)
                  opts = int_model._meta
                  path.append(PathInfo(
                      from_opts=final_field.model._meta,
                      to_opts=opts,
                      target_fields=targets,
                      join_field=final_field,
                      m2m=False,
                      direct=True,
                      filtered_relation=None,
                  ))
          return path
  
      def get_path_from_parent(self, parent):
          """
          Return a list of PathInfos containing the path from the parent
          model to the current model, or an empty list if parent is not a
          parent of the current model.
          """
          if self.model is parent:
              return []
          model = self.concrete_model
          # Get a reversed base chain including both the current and parent
          # models.
          chain = model._meta.get_base_chain(parent)
          chain.reverse()
          chain.append(model)
          # Construct a list of the PathInfos between models in chain.
          path = []
          for i, ancestor in enumerate(chain[:-1]):
              child = chain[i + 1]
              link = child._meta.get_ancestor_link(ancestor)
              path.extend(link.get_reverse_path_info())
          return path
  
      def _populate_directed_relation_graph(self):
          """
          This method is used by each model to find its reverse objects. As this
          method is very expensive and is accessed frequently (it looks up every
          field in a model, in every app), it is computed on first access and then
          is set as a property on every model.
          """
          related_objects_graph = defaultdict(list)
  
          all_models = self.apps.get_models(include_auto_created=True)
          for model in all_models:
              opts = model._meta
              # Abstract model's fields are copied to child models, hence we will
              # see the fields from the child models.
              if opts.abstract:
                  continue
              fields_with_relations = (
                  f for f in opts._get_fields(reverse=False, include_parents=False)
                  if f.is_relation and f.related_model is not None
              )
              for f in fields_with_relations:
                  if not isinstance(f.remote_field.model, str):
                      remote_label = f.remote_field.model._meta.concrete_model._meta.label
                      related_objects_graph[remote_label].append(f)
  
          for model in all_models:
              # Set the relation_tree using the internal __dict__. In this way
              # we avoid calling the cached property. In attribute lookup,
              # __dict__ takes precedence over a data descriptor (such as
              # @cached_property). This means that the _meta._relation_tree is
              # only called if related_objects is not in __dict__.
              related_objects = related_objects_graph[model._meta.concrete_model._meta.label]
              model._meta.__dict__['_relation_tree'] = related_objects
          # It seems it is possible that self is not in all_models, so guard
          # against that with default for get().
          return self.__dict__.get('_relation_tree', EMPTY_RELATION_TREE)
  
      @cached_property
      def _relation_tree(self):
          return self._populate_directed_relation_graph()
  
      def _expire_cache(self, forward=True, reverse=True):
          # This method is usually called by apps.cache_clear(), when the
          # registry is finalized, or when a new field is added.
          if forward:
              for cache_key in self.FORWARD_PROPERTIES:
                  if cache_key in self.__dict__:
                      delattr(self, cache_key)
          if reverse and not self.abstract:
              for cache_key in self.REVERSE_PROPERTIES:
                  if cache_key in self.__dict__:
                      delattr(self, cache_key)
          self._get_fields_cache = {}
  
      def get_fields(self, include_parents=True, include_hidden=False):
          """
          Return a list of fields associated to the model. By default, include
          forward and reverse fields, fields derived from inheritance, but not
          hidden fields. The returned fields can be changed using the parameters:
  
          - include_parents: include fields derived from inheritance
          - include_hidden:  include fields that have a related_name that
                             starts with a "+"
          """
          if include_parents is False:
              include_parents = PROXY_PARENTS
          return self._get_fields(include_parents=include_parents, include_hidden=include_hidden)
  
      def _get_fields(self, forward=True, reverse=True, include_parents=True, include_hidden=False,
                      seen_models=None):
          """
          Internal helper function to return fields of the model.
          * If forward=True, then fields defined on this model are returned.
          * If reverse=True, then relations pointing to this model are returned.
          * If include_hidden=True, then fields with is_hidden=True are returned.
          * The include_parents argument toggles if fields from parent models
            should be included. It has three values: True, False, and
            PROXY_PARENTS. When set to PROXY_PARENTS, the call will return all
            fields defined for the current model or any of its parents in the
            parent chain to the model's concrete model.
          """
          if include_parents not in (True, False, PROXY_PARENTS):
              raise TypeError("Invalid argument for include_parents: %s" % (include_parents,))
          # This helper function is used to allow recursion in ``get_fields()``
          # implementation and to provide a fast way for Django's internals to
          # access specific subsets of fields.
  
          # We must keep track of which models we have already seen. Otherwise we
          # could include the same field multiple times from different models.
          topmost_call = seen_models is None
          if topmost_call:
              seen_models = set()
          seen_models.add(self.model)
  
          # Creates a cache key composed of all arguments
          cache_key = (forward, reverse, include_parents, include_hidden, topmost_call)
  
          try:
              # In order to avoid list manipulation. Always return a shallow copy
              # of the results.
              return self._get_fields_cache[cache_key]
          except KeyError:
              pass
  
          fields = []
          # Recursively call _get_fields() on each parent, with the same
          # options provided in this call.
          if include_parents is not False:
              for parent in self.parents:
                  # In diamond inheritance it is possible that we see the same
                  # model from two different routes. In that case, avoid adding
                  # fields from the same parent again.
                  if parent in seen_models:
                      continue
                  if (parent._meta.concrete_model != self.concrete_model and
                          include_parents == PROXY_PARENTS):
                      continue
                  for obj in parent._meta._get_fields(
                          forward=forward, reverse=reverse, include_parents=include_parents,
                          include_hidden=include_hidden, seen_models=seen_models):
                      if not getattr(obj, 'parent_link', False) or obj.model == self.concrete_model:
                          fields.append(obj)
          if reverse and not self.proxy:
              # Tree is computed once and cached until the app cache is expired.
              # It is composed of a list of fields pointing to the current model
              # from other models.
              all_fields = self._relation_tree
              for field in all_fields:
                  # If hidden fields should be included or the relation is not
                  # intentionally hidden, add to the fields dict.
                  if include_hidden or not field.remote_field.hidden:
                      fields.append(field.remote_field)
  
          if forward:
              fields += self.local_fields
              fields += self.local_many_to_many
              # Private fields are recopied to each child model, and they get a
              # different model as field.model in each child. Hence we have to
              # add the private fields separately from the topmost call. If we
              # did this recursively similar to local_fields, we would get field
              # instances with field.model != self.model.
              if topmost_call:
                  fields += self.private_fields
  
          # In order to avoid list manipulation. Always
          # return a shallow copy of the results
          fields = make_immutable_fields_list("get_fields()", fields)
  
          # Store result into cache for later access
          self._get_fields_cache[cache_key] = fields
          return fields
  
      @cached_property
      def total_unique_constraints(self):
          """
          Return a list of total unique constraints. Useful for determining set
          of fields guaranteed to be unique for all rows.
          """
          return [
              constraint
              for constraint in self.constraints
              if isinstance(constraint, UniqueConstraint) and constraint.condition is None
          ]
  
      @cached_property
      def _property_names(self):
          """Return a set of the names of the properties defined on the model."""
          names = []
          for name in dir(self.model):
              attr = inspect.getattr_static(self.model, name)
              if isinstance(attr, property):
                  names.append(name)
          return frozenset(names)
  
      @cached_property
      def db_returning_fields(self):
          """
          Private API intended only to be used by Django itself.
          Fields to be returned after a database insert.
          """
          return [
              field for field in self._get_fields(forward=True, reverse=False, include_parents=PROXY_PARENTS)
              if getattr(field, 'db_returning', False)
          ]
  