'Return all fields on this Serializer class which are read-only.
Expects sub-classes implement Meta.explicit_read_only_fields,
which is a tuple declaring read-only fields which were declared
explicitly and thus could not be added to the usual
cls.Meta.read_only_fields tuple.'

@classmethod
def get_read_only_fields(cls):

    return (getattr(cls.Meta, 'read_only_fields', '') + getattr(cls.Meta, 'explicit_read_only_fields', ''))
