docs: def _get_unique_constraints(self, instance):
          """Return SQL filter for filtering by fields in ``unique_with`` attribute.
  
          Filter is returned as tuple of two elements where first one is
          placeholder which is safe to insert into SQL query and second
          one may include potentially dangerous values and must be passed
          to SQL query in ``params`` attribute to make sure it is properly
          escaped.
          """
          constraints_expression = []
          constraints_values = {}
          for field_name in self.unique_with:
              if constants.LOOKUP_SEP in field_name:
                  raise NotImplementedError(
                      '`unique_with` constraint does not support lookups by related models.'
                  )
  
              field = instance._meta.get_field(field_name)  # pylint: disable=protected-access
              field_value = getattr(instance, field_name)
  
              # Convert value to the database representation.
              field_db_value = field.get_prep_value(field_value)
  
              constraint_key = 'unique_' + field_name
              constraints_expression.append("{} = %({})s".format(
                  connection.ops.quote_name(field.column),
                  constraint_key
              ))
              constraints_values[constraint_key] = field_db_value
  
          if not constraints_expression:
              return '', []
  
          constraints_expression = 'AND ' + ' AND '.join(constraints_expression)
          return constraints_expression, constraints_values