Refactor Association into Field like everything else (#1897)

* Make assocations asserts easier to understand

* Refactor Association into Field like everything else

* Make assocation serializer/links/meta lazier

* Push association deeper into relationship

* Simplify association usage in relationships

* Better naming of reflection parent serializer

* Easier to read association method

lib/active_model/serializer/association.rb

@@ -3,17 +3,32 @@ module ActiveModel
     # This class hold all information about serializer's association.
     #
     # @attr [Symbol] name
-    # @attr [ActiveModel::Serializer] serializer
     # @attr [Hash{Symbol => Object}] options
+    # @attr [block]
     #
     # @example
-    #  Association.new(:comments, CommentSummarySerializer)
+    #  Association.new(:comments, { serializer: CommentSummarySerializer })
     #
-    Association = Struct.new(:name, :serializer, :options, :links, :meta) do
+    class Association < Field
       # @return [Symbol]
       def key
         options.fetch(:key, name)
       end
+
+      # @return [ActiveModel::Serializer, nil]
+      def serializer
+        options[:serializer]
+      end
+
+      # @return [Hash]
+      def links
+        options.fetch(:links) || {}
+      end
+
+      # @return [Hash, nil]
+      def meta
+        options[:meta]
+      end
     end
   end
 end

lib/active_model/serializer/concerns/associations.rb

@@ -74,8 +74,8 @@ module ActiveModel
         # @api private
         #
         def associate(reflection)
-          key = reflection.options[:key]
-          key ? self._reflections[key] = reflection : self._reflections[reflection.name] = reflection
+          key = reflection.options[:key] || reflection.name
+          self._reflections[key] = reflection
         end
       end
 

lib/active_model/serializer/reflection.rb

@@ -88,7 +88,7 @@ module ActiveModel
       # Build association. This method is used internally to
       # build serializer's association by its reflection.
       #
-      # @param [Serializer] subject is a parent serializer for given association
+      # @param [Serializer] parent_serializer for given association
       # @param [Hash{Symbol => Object}] parent_serializer_options
       #
       # @example
@@ -106,17 +106,19 @@ module ActiveModel
       #
       # @api private
       #
-      def build_association(subject, parent_serializer_options)
-        association_value = value(subject)
+      def build_association(parent_serializer, parent_serializer_options)
+        association_value = value(parent_serializer)
         reflection_options = options.dup
-        serializer_class = subject.class.serializer_for(association_value, reflection_options)
+        serializer_class = parent_serializer.class.serializer_for(association_value, reflection_options)
         reflection_options[:include_data] = @_include_data
+        reflection_options[:links] = @_links
+        reflection_options[:meta] = @_meta
 
         if serializer_class
           begin
-            serializer = serializer_class.new(
+            reflection_options[:serializer] = serializer_class.new(
               association_value,
-              serializer_options(subject, parent_serializer_options, reflection_options)
+              serializer_options(parent_serializer, parent_serializer_options, reflection_options)
             )
           rescue ActiveModel::Serializer::CollectionSerializer::NoSerializerError
             reflection_options[:virtual_value] = association_value.try(:as_json) || association_value
@@ -125,7 +127,8 @@ module ActiveModel
           reflection_options[:virtual_value] = association_value
         end
 
-        Association.new(name, serializer, reflection_options, @_links, @_meta)
+        block = nil
+        Association.new(name, reflection_options, block)
       end
 
       protected
@@ -134,12 +137,12 @@ module ActiveModel
 
       private
 
-      def serializer_options(subject, parent_serializer_options, reflection_options)
+      def serializer_options(parent_serializer, parent_serializer_options, reflection_options)
         serializer = reflection_options.fetch(:serializer, nil)
 
         serializer_options = parent_serializer_options.except(:serializer)
         serializer_options[:serializer] = serializer if serializer
-        serializer_options[:serializer_context_class] = subject.class
+        serializer_options[:serializer_context_class] = parent_serializer.class
         serializer_options
       end
     end