Added get_related_field_name method to RelationshipView

* Added get_related_field_name method to RelationshipView

* Added docs about field_name_mapping

Author: Anton-Shutik

docs/usage.md

@@ -413,6 +413,15 @@ model field `Order.line_items` on the Order with pk 3, the url would be
 `self_link_view_name` keyword argument, which should match the `name=`
 provided in the urlconf, and will use the name of the field for the
 `related_field` kwarg.
+Also we can override `related_field` in the url. Let's say we want the url to be:
+`/order/3/relationships/order_items` - all we need to do is just add `field_name_mapping`
+dict to the class:
+```python
+field_name_mapping = {
+        'line_items': 'order_items'
+    }
+```
+
 
 ### Meta
 

rest_framework_json_api/views.py

@@ -19,6 +19,7 @@ class RelationshipView(generics.GenericAPIView):
     serializer_class = ResourceIdentifierObjectSerializer
     self_link_view_name = None
     related_link_view_name = None
+    field_name_mapping = {}
 
     def get_serializer_class(self):
         if getattr(self, 'action', False) is None:
@@ -96,7 +97,7 @@ def patch(self, request, *args, **kwargs):
             related_model_class = related_instance_or_manager.__class__
             serializer = self.get_serializer(data=request.data, model_class=related_model_class)
             serializer.is_valid(raise_exception=True)
-            setattr(parent_obj, kwargs['related_field'], serializer.validated_data)
+            setattr(parent_obj, self.get_related_field_name(), serializer.validated_data)
             parent_obj.save()
         result_serializer = self._instantiate_serializer(related_instance_or_manager)
         return Response(result_serializer.data)
@@ -138,10 +139,16 @@ def delete(self, request, *args, **kwargs):
 
     def get_related_instance(self):
         try:
-            return getattr(self.get_object(), self.kwargs['related_field'])
+            return getattr(self.get_object(), self.get_related_field_name())
         except AttributeError:
             raise NotFound
 
+    def get_related_field_name(self):
+        field_name = self.kwargs['related_field']
+        if field_name in self.field_name_mapping:
+            return self.field_name_mapping[field_name]
+        return field_name
+
     def _instantiate_serializer(self, instance):
         if isinstance(instance, Model) or instance is None:
             return self.get_serializer(instance=instance)
@@ -153,7 +160,7 @@ def _instantiate_serializer(self, instance):
 
     def get_resource_name(self):
         if not hasattr(self, '_resource_name'):
-            instance = getattr(self.get_object(), self.kwargs['related_field'])
+            instance = getattr(self.get_object(), self.get_related_field_name())
             self._resource_name = get_resource_type_from_instance(instance)
         return self._resource_name