There is an Implicit assumption that the serializer in an expandable field definition subclasses ModelSerializer

[gareth-lloyd]

To reproduce:

from rest_framework import serializers
from rest_framework_serializer_extensions import serializers as extension_serializers

class MyChildSerializer(serializers.Serializer):
    pass

class MyParentSerializer(extension_serializers.ExpandableFieldsMixin, serializers.Serializer):
    class Meta:
        expandable_fields = {
            "child": MyChildSerializer
        }

MyParentSerializer()

Result

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-3-174f3862f0ed> in <module>()
----> 1 MyParentSerializer()

/home/vagrant/.virtualenvs/housekeep/local/lib/python2.7/site-packages/rest_framework_serializer_extensions/serializers.pyc in __init__(self, *args, **kwargs)
    157         super(ExpandableFieldsMixin, self).__init__(*args, **kwargs)
    158         self.expandable_fields = self._standardise_expandable_definitions(
--> 159             self.get_expandable_field_definitions()
    160         )
    161

/home/vagrant/.virtualenvs/housekeep/local/lib/python2.7/site-packages/rest_framework_serializer_extensions/serializers.pyc in _standardise_expandable_definitions(self, expandable_fields)
    188         return {
    189             key: self._standardise_expandable_definition(definition)
--> 190             for key, definition in six.iteritems(expandable_fields)
    191         }
    192

/home/vagrant/.virtualenvs/housekeep/local/lib/python2.7/site-packages/rest_framework_serializer_extensions/serializers.pyc in <dictcomp>((key, definition))
    188         return {
    189             key: self._standardise_expandable_definition(definition)
--> 190             for key, definition in six.iteritems(expandable_fields)
    191         }
    192

/home/vagrant/.virtualenvs/housekeep/local/lib/python2.7/site-packages/rest_framework_serializer_extensions/serializers.pyc in _standardise_expandable_definition(self, definition)
    214         # The model used for the HashId (defaults to the serializer's model)
    215         if 'id_model' not in definition:
--> 216             definition['id_model'] = definition['serializer'].Meta.model
    217
    218         # read_only defaults to True

AttributeError: type object 'MyChildSerializer' has no attribute 'Meta'

There is a workaround: specify id_model in the expandable field definition with a value of None.

class MyParentSerializer(extension_serializers.ExpandableFieldsMixin, serializers.Serializer):
    class Meta:
        expandable_fields = {
            "child": {
                "serializer": MyChildSerializer,
                "id_model": None,
        }