Fix 0.10.6 regression; make using belongs_to on self opt-in

Author: bf4

docs/general/configuration_options.md

@@ -57,6 +57,7 @@ still prefer the render option `:key_transform` over this setting.
 application, setting `config.key_transform` to `:unaltered` will provide a performance boost.*
 
 ##### default_includes
+
 What relationships to serialize by default.  Default: `'*'`, which includes one level of related
 objects. See [includes](adapters.md#included) for more info.
 
@@ -162,6 +163,21 @@ Default: `{}`.
 
 *Used when `jsonapi_include_toplevel_object` is `true`*
 
+##### jsonapi_use_foreign_key_on_belongs_to_relationship
+
+When true, the relationship will determine its resource object identifier
+without calling the association or its serializer.  This can be useful when calling
+the association object is triggering unnecessary queries.
+
+For example, if a `comment` belongs to a `post`, and the comment
+uses the foreign key `post_id`, we can determine the resource object
+identifier `id` as `comment.post_id` and the `type` from the association options.
+Or quite simply, it behaves as `belongs_to :post, type: :posts, foreign_key: :post_id`.
+
+Note: This option has *no effect* on polymorphic associations as we cannot reliably
+determine the associated object's type without instantiating it.
+
+Default: `false`.
 
 ## Hooks
 

lib/active_model/serializer.rb

@@ -142,6 +142,7 @@ def config.array_serializer
     # Make JSON API top-level jsonapi member opt-in
     # ref: http://jsonapi.org/format/#document-top-level
     config.jsonapi_include_toplevel_object = false
+    config.jsonapi_use_foreign_key_on_belongs_to_relationship = false
     config.include_data_default = true
 
     # For configuring how serializers are found.

lib/active_model_serializers/adapter/json_api/relationship.rb

@@ -94,7 +94,8 @@ def meta_for(association)
         end
 
         def belongs_to_id_on_self?(association)
-          association.belongs_to? &&
+          parent_serializer.config.jsonapi_use_foreign_key_on_belongs_to_relationship &&
+            association.belongs_to? &&
             parent_serializer.object.respond_to?(association.reflection.foreign_key)
         end
       end