def _get_model_fields(self, field_names, declared_fields, extra_kwargs):
        """
        Returns all the model fields that are being mapped to by fields
        on the serializer class.
        Returned as a dict of 'model field name' -> 'model field'.
        Used internally by `get_uniqueness_field_options`.
        """
        model = getattr(self.Meta, 'model')
        model_fields = {}

        for field_name in field_names:
            if field_name in declared_fields:
                # If the field is declared on the serializer
                field = declared_fields[field_name]
                source = field.source or field_name
            else:
                try:
                    source = extra_kwargs[field_name]['source']
                except KeyError:
                    source = field_name

            if '.' in source or source == '*':
                # Model fields will always have a simple source mapping,
                # they can't be nested attribute lookups.
                continue

            try:
                field = model._meta.get_field(source)
                if isinstance(field, DjangoModelField):
                    model_fields[source] = field
            except FieldDoesNotExist:
                pass

        return model_fields
Returns all the model fields that are being mapped to by fields
        on the serializer class.
        Returned as a dict of 'model field name' -> 'model field'.
        Used internally by `get_uniqueness_field_options`.