Polymorphic ancestors must now be defined in Django's settings

Update documentation

Author: leo-naeka

docs/usage.md

@@ -248,6 +248,19 @@ When set to pluralize:
 Both `JSON_API_PLURALIZE_RELATION_TYPE` and `JSON_API_FORMAT_RELATION_KEYS` can be combined to
 achieve different results.
 
+### Working with polymorphic resources
+
+This package can defer the resolution of the type of polymorphic models instances to get the appropriate type.
+However, most models are not polymorphic and for performance reasons this is only done if the underlying model is a subclass of a polymorphic model.
+
+Polymorphic ancestors must be defined on settings like this:
+
+```python
+JSON_API_POLYMORPHIC_ANCESTORS = (
+    'polymorphic.models.PolymorphicModel',
+)
+```
+
 ### Meta
 
 You may add metadata to the rendered json in two different ways: `meta_fields` and `get_root_meta`.

example/settings/test.py

@@ -15,3 +15,6 @@
 REST_FRAMEWORK.update({
     'PAGE_SIZE': 1,
 })
+JSON_API_POLYMORPHIC_ANCESTORS = (
+    'polymorphic.models.PolymorphicModel',
+)

rest_framework_json_api/utils.py

@@ -22,16 +22,9 @@
     HyperlinkedRouterField = type(None)
 
 POLYMORPHIC_ANCESTORS = ()
-try:
-    from polymorphic.models import PolymorphicModel
-    POLYMORPHIC_ANCESTORS += (PolymorphicModel,)
-except ImportError:
-    pass
-try:
-    from typedmodels.models import TypedModel
-    POLYMORPHIC_ANCESTORS += (TypedModel,)
-except ImportError:
-    pass
+for ancestor in getattr(settings, 'JSON_API_POLYMORPHIC_ANCESTORS', ()):
+    ancestor_class = import_class_from_dotted_path(ancestor)
+    POLYMORPHIC_ANCESTORS += (ancestor_class,)
 
 
 def get_resource_name(context):