Partially fix extra db hit with belongs_to.

lib/active_model/serializer/adapter/json_api.rb

@@ -62,6 +62,16 @@ def resource_identifier_type_for(serializer)
           end
         end
 
+        def resource_identifier_type_for_belongs_to(parent_object, association_key)
+          type = parent_object.class.reflections[association_key].class_name
+          if ActiveModel::Serializer.config.jsonapi_resource_type == :singular
+            type = type.singularize
+          else
+            type = type.pluralize
+          end
+          type.underscore
+        end
+
         def resource_identifier_id_for(serializer)
           if serializer.respond_to?(:id)
             serializer.id
@@ -70,13 +80,24 @@ def resource_identifier_id_for(serializer)
           end
         end
 
+        def resource_identifier_id_for_belongs_to(parent_object, association_key)
+          parent_object.send("#{association_key}_id")
+        end
+
         def resource_identifier_for(serializer)
           type = resource_identifier_type_for(serializer)
           id   = resource_identifier_id_for(serializer)
 
           { id: id.to_s, type: type }
         end
 
+        def resource_identifier_for_belongs_to(parent_object, association_key)
+          type = resource_identifier_type_for_belongs_to(parent_object, association_key)
+          id   = resource_identifier_id_for_belongs_to(parent_object, association_key)
+
+          { id: id.to_s, type: type } unless id.nil?
+        end
+
         def resource_object_for(serializer, options = {})
           options[:fields] = @fieldset && @fieldset.fields_for(serializer)
 
@@ -96,20 +117,22 @@ def primary_data_for(serializer, options)
           end
         end
 
-        def relationship_value_for(serializer, options = {})
-          if serializer.respond_to?(:each)
-            serializer.map { |s| resource_identifier_for(s) }
+        def relationship_value_for(parent_object, association)
+          if association.serializer.respond_to?(:each)
+            association.serializer.map { |serializer| resource_identifier_for(serializer) }
           else
-            if options[:virtual_value]
-              options[:virtual_value]
-            elsif serializer && serializer.object
-              resource_identifier_for(serializer)
+            if association.options[:virtual_value]
+              association.options[:virtual_value]
+            elsif parent_object.class.respond_to?(:reflections) && parent_object.class.reflections[association.key.to_s].belongs_to?
+              resource_identifier_for_belongs_to(parent_object, association.key.to_s)
+            elsif association.serializer && association.serializer.object
+              resource_identifier_for(association.serializer)
             end
           end
         end
 
         def relationships_for(serializer)
-          Hash[serializer.associations.map { |association| [association.key, { data: relationship_value_for(association.serializer, association.options) }] }]
+          Hash[serializer.associations.map { |association| [association.key, { data: relationship_value_for(serializer.object, association) }] }]
         end
 
         def included_for(serializer)

test/adapter/json_api/belongs_to_test.rb

@@ -150,6 +150,27 @@ def test_include_linked_resources_with_type_name
             ]
             assert_equal expected, linked
           end
+
+          def test_ar_belongs_to_doesnt_load_record
+            author = ARModels::User.create(name: 'Name 1')
+            post = ARModels::Post.create(author_id: author.id)
+
+            class << post
+              def author
+                fail "should use author_id"
+              end
+            end
+
+            hash = ActiveModel::SerializableResource.new(post, adapter: :json_api).serializable_hash
+            assert_equal({ data: { id: author.id.to_s, type: 'users' } }, hash[:data][:relationships][:author])
+          end
+
+          def test_ar_belongs_to_empty
+            post = ARModels::Post.create
+
+            hash = ActiveModel::SerializableResource.new(post, adapter: :json_api).serializable_hash
+            assert_equal({ data: nil }, hash[:data][:relationships][:author])
+          end
         end
       end
     end

test/fixtures/active_record.rb

@@ -8,7 +8,7 @@
     t.references :author
     t.timestamps null: false
   end
-  create_table :authors, force: true do |t|
+  create_table :users, force: true do |t|
     t.string :name
     t.timestamps null: false
   end
@@ -23,15 +23,15 @@
 module ARModels
   class Post < ActiveRecord::Base
     has_many :comments
-    belongs_to :author
+    belongs_to :author, class_name: 'User'
   end
 
   class Comment < ActiveRecord::Base
     belongs_to :post
-    belongs_to :author
+    belongs_to :author, class_name: 'User'
   end
 
-  class Author < ActiveRecord::Base
+  class User < ActiveRecord::Base
     has_many :posts
   end
 
@@ -49,7 +49,7 @@ class CommentSerializer < ActiveModel::Serializer
     belongs_to :author
   end
 
-  class AuthorSerializer < ActiveModel::Serializer
+  class UserSerializer < ActiveModel::Serializer
     attributes :id, :name
 
     has_many :posts