text: from django.db.models import Func, Transform, Value, fields
  from django.db.models.functions import Coalesce
  
  
  class ConcatPair(Func):
      """
      Concatenate two arguments together. This is used by `Concat` because not
      all backend databases support more than two arguments.
      """
      function = 'CONCAT'
  
      def as_sqlite(self, compiler, connection):
          coalesced = self.coalesce()
          return super(ConcatPair, coalesced).as_sql(
              compiler, connection, template='%(expressions)s', arg_joiner=' || '
          )
  
      def as_mysql(self, compiler, connection):
          # Use CONCAT_WS with an empty separator so that NULLs are ignored.
          return super().as_sql(
              compiler, connection, function='CONCAT_WS', template="%(function)s('', %(expressions)s)"
          )
  
      def coalesce(self):
          # null on either side results in null for expression, wrap with coalesce
          c = self.copy()
          expressions = [
              Coalesce(expression, Value('')) for expression in c.get_source_expressions()
          ]
          c.set_source_expressions(expressions)
          return c
  
  
  class Concat(Func):
      """
      Concatenate text fields together. Backends that result in an entire
      null expression when any arguments are null will wrap each argument in
      coalesce functions to ensure a non-null result.
      """
      function = None
      template = "%(expressions)s"
  
      def __init__(self, *expressions, **extra):
          if len(expressions) < 2:
              raise ValueError('Concat must take at least two expressions')
          paired = self._paired(expressions)
          super().__init__(paired, **extra)
  
      def _paired(self, expressions):
          # wrap pairs of expressions in successive concat functions
          # exp = [a, b, c, d]
          # -> ConcatPair(a, ConcatPair(b, ConcatPair(c, d))))
          if len(expressions) == 2:
              return ConcatPair(*expressions)
          return ConcatPair(expressions[0], self._paired(expressions[1:]))
  
  
  class Length(Transform):
      """Return the number of characters in the expression."""
      function = 'LENGTH'
      lookup_name = 'length'
      output_field = fields.IntegerField()
  
      def as_mysql(self, compiler, connection):
          return super().as_sql(compiler, connection, function='CHAR_LENGTH')
  
  
  class Lower(Transform):
      function = 'LOWER'
      lookup_name = 'lower'
  
  
  class Replace(Func):
      function = 'REPLACE'
  
      def __init__(self, expression, text, replacement=Value(''), **extra):
          super().__init__(expression, text, replacement, **extra)
  
  
  class StrIndex(Func):
      """
      Return a positive integer corresponding to the 1-indexed position of the
      first occurrence of a substring inside another string, or 0 if the
      substring is not found.
      """
      function = 'INSTR'
      arity = 2
      output_field = fields.IntegerField()
  
      def as_postgresql(self, compiler, connection):
          return super().as_sql(compiler, connection, function='STRPOS')
  
  
  class Substr(Func):
      function = 'SUBSTRING'
  
      def __init__(self, expression, pos, length=None, **extra):
          """
          expression: the name of a field, or an expression returning a string
          pos: an integer > 0, or an expression returning an integer
          length: an optional number of characters to return
          """
          if not hasattr(pos, 'resolve_expression'):
              if pos < 1:
                  raise ValueError("'pos' must be greater than 0")
          expressions = [expression, pos]
          if length is not None:
              expressions.append(length)
          super().__init__(*expressions, **extra)
  
      def as_sqlite(self, compiler, connection):
          return super().as_sql(compiler, connection, function='SUBSTR')
  
      def as_oracle(self, compiler, connection):
          return super().as_sql(compiler, connection, function='SUBSTR')
  
  
  class Upper(Transform):
      function = 'UPPER'
      lookup_name = 'upper'
  