Use foreign_key option when getting foreign key value.

Author: dougo

lib/jsonapi/resource_serializer.rb

@@ -454,11 +454,11 @@ def link_object(source, relationship, include_linkage = false)
     def foreign_key_value(source, relationship)
       related_resource_id = if source.preloaded_fragments.has_key?(format_key(relationship.name))
         source.preloaded_fragments[format_key(relationship.name)].values.first.try(:id)
-      elsif source.respond_to?("#{relationship.name}_id")
+      elsif source.respond_to?(relationship.foreign_key)
         # If you have direct access to the underlying id, you don't have to load the relationship
         # which can save quite a lot of time when loading a lot of data.
         # This does not apply to e.g. has_one :through relationships.
-        source.public_send("#{relationship.name}_id")
+        source.public_send(relationship.foreign_key)
       else
         source.public_send(relationship.name).try(:id)
       end

test/unit/serializer/serializer_test.rb

@@ -985,6 +985,18 @@ def test_serializer_array_of_resources_always_include_to_one_linkage_data
     JSONAPI.configuration.always_include_to_one_linkage_data = false
   end
 
+  def test_serializer_always_include_to_one_linkage_data_does_not_load_association
+    JSONAPI.configuration.always_include_to_one_linkage_data = true
+
+    post = Post.find(1)
+    resource = Api::V1::PostResource.new(post, nil)
+    JSONAPI::ResourceSerializer.new(Api::V1::PostResource).serialize_to_hash(resource)
+
+    refute_predicate post.association(:writer), :loaded?
+  ensure
+    JSONAPI.configuration.always_include_to_one_linkage_data = false
+  end
+
   def test_serializer_array_of_resources
 
     posts = []