Improve performance of extract_related()

Check more thoroughly if the related resource is already included
in the response, and skip serialization if it is.

Author: juyrjola

AUTHORS

@@ -23,3 +23,4 @@ Mohammed Ali Zubair <mazg1493@gmail.com>
 Jason Housley <housleyjk@gmail.com>
 Beni Keller <beni@matraxi.ch>
 Stas S. <stas@nerd.ro>
+Juha Yrjölä <juha.yrjola@iki.fi>

CHANGELOG.md

@@ -31,6 +31,7 @@ any parts of the framework not mentioned in the documentation should generally b
 * Allow `HyperlinkRelatedField` to be used with [related urls](https://django-rest-framework-json-api.readthedocs.io/en/stable/usage.html?highlight=related%20links#related-urls)
 * Avoid exception in `AutoPrefetchMixin` when including a reverse one to one relation ([#537](https://github.com/django-json-api/django-rest-framework-json-api/issues/537))
 * Avoid requested resource(s) to be added to included as well ([#524](https://github.com/django-json-api/django-rest-framework-json-api/issues/524))
+* Performance improvement when rendering included data
 
 ## [2.6.0] - 2018-09-20
 

rest_framework_json_api/renderers.py

@@ -408,6 +408,10 @@ def extract_included(cls, fields, resource, resource_instance, included_resource
                             relation_type or
                             utils.get_resource_type_from_instance(nested_resource_instance)
                         )
+                        resource_id = encoding.force_text(nested_resource_instance.pk)
+                        if included_cache.get(resource_type, {}).get(resource_id):
+                            # Do not serialize if already included in the response
+                            continue
                         serializer_fields = utils.get_serializer_fields(
                             serializer.__class__(
                                 nested_resource_instance, context=serializer.context
@@ -432,7 +436,10 @@ def extract_included(cls, fields, resource, resource_instance, included_resource
 
             if isinstance(field, Serializer):
                 relation_type = utils.get_resource_type_from_serializer(field)
-
+                resource_id = encoding.force_text(relation_instance.pk)
+                if included_cache.get(relation_type, {}).get(resource_id):
+                    # Do not serialize if already included in the response
+                    continue
                 # Get the serializer fields
                 serializer_fields = utils.get_serializer_fields(field)
                 if serializer_data: