Fix for apps that don't load contenttypes module.

PR django-json-api#319 brought support for generic relations. Unfortunately apps that
don't add contenttypes to it's INSTALLED_APPS would crash and burn:

```
RuntimeError: Model class django.contrib.contenttypes.models.ContentType doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS.
```

Also using `type(something) is object()` comparison as a safer
alternative to `type(something) is type(None)`. If `something` happened
to be `None` we would enter a branch that was never supposed to run.

Author: amw

CHANGELOG.md

@@ -3,6 +3,7 @@ v2.3.0
 * When `JSON_API_FORMAT_KEYS` is False (the default) do not translate request
   attributes and relations to snake\_case format. This conversion was unexpected
   and there was no way to turn it off.
+* Fix for apps that don't use `django.contrib.contenttypes`.
 
 v2.2.0
 

rest_framework_json_api/utils.py

@@ -20,21 +20,28 @@
 try:
     from rest_framework.serializers import ManyRelatedField
 except ImportError:
-    ManyRelatedField = type(None)
+    ManyRelatedField = object()
 
 try:
     from rest_framework_nested.relations import HyperlinkedRouterField
 except ImportError:
-    HyperlinkedRouterField = type(None)
+    HyperlinkedRouterField = object()
 
 if django.VERSION >= (1, 9):
     from django.db.models.fields.related_descriptors import ManyToManyDescriptor, ReverseManyToOneDescriptor
-    ReverseManyRelatedObjectsDescriptor = type(None)
-    from django.contrib.contenttypes.fields import ReverseGenericManyToOneDescriptor
+    ReverseManyRelatedObjectsDescriptor = object()
 else:
     from django.db.models.fields.related import ManyRelatedObjectsDescriptor as ManyToManyDescriptor
     from django.db.models.fields.related import ForeignRelatedObjectsDescriptor as ReverseManyToOneDescriptor
     from django.db.models.fields.related import ReverseManyRelatedObjectsDescriptor
+
+# Generic relation descriptor from django.contrib.contenttypes.
+if 'django.contrib.contenttypes' not in settings.INSTALLED_APPS:
+    # Target application does not use contenttypes. Importing would cause errors.
+    ReverseGenericManyToOneDescriptor = object()
+elif django.VERSION >= (1, 9):
+    from django.contrib.contenttypes.fields import ReverseGenericManyToOneDescriptor
+else:
     from django.contrib.contenttypes.fields import ReverseGenericRelatedObjectsDescriptor as ReverseGenericManyToOneDescriptor