Make Association totally lazy

lib/active_model/serializer/lazy_association.rb

@@ -1,21 +1,107 @@
 module ActiveModel
   class Serializer
-    class LazyAssociation < Field
+    # @api private
+    LazyAssociation = Struct.new(:reflection, :association_options) do
+      REFLECTION_OPTIONS = %i(key links polymorphic meta serializer virtual_value namespace).freeze
 
-      def serializer
-        options[:serializer]
+      delegate :collection?, to: :reflection
+
+      def reflection_options
+        @reflection_options ||= reflection.options.dup.reject { |k, _| !REFLECTION_OPTIONS.include?(k) }
       end
 
+      def object
+        @object ||= reflection.value(
+          association_options.fetch(:parent_serializer),
+          association_options.fetch(:include_slice)
+        )
+      end
+      alias_method :eval_reflection_block, :object
+
       def include_data?
-        options[:include_data]
+        eval_reflection_block if reflection.block
+        reflection.include_data?(
+          association_options.fetch(:include_slice)
+        )
+      end
+
+      # @return [ActiveModel::Serializer, nil]
+      def serializer
+        return @serializer if defined?(@serializer)
+        if serializer_class
+          serialize_object!(object)
+        elsif !object.nil? && !object.instance_of?(Object)
+          cached_result[:virtual_value] = object
+        end
+        @serializer = cached_result[:serializer]
       end
 
       def virtual_value
-        options[:virtual_value]
+        cached_result[:virtual_value] || reflection_options[:virtual_value]
       end
 
-      def reflection
-        options[:reflection]
+      # NOTE(BF): Kurko writes:
+      # 1. This class is doing a lot more than it should. It has business logic (key/meta/links) and
+      #   it also looks like a factory (serializer/serialize_object/instantiate_serializer/serializer_class).
+      #   It's hard to maintain classes that you can understand what it's really meant to be doing,
+      #   so it ends up having all sorts of methods.
+      #   Perhaps we could replace all these methods with a class called... Serializer.
+      #   See how association is doing the job a serializer again?
+      # 2. I've seen code like this in many other places.
+      #   Perhaps we should just have it all in one place: Serializer.
+      #   We already have a class called Serializer, I know,
+      #   and that is doing things that are not responsibility of a serializer.
+      def serializer_class
+        return @serializer_class if defined?(@serializer_class)
+        serializer_for_options = { namespace: namespace }
+        serializer_for_options[:serializer] = reflection_options[:serializer] if reflection_options.key?(:serializer)
+        @serializer_class = association_options.fetch(:parent_serializer).class.serializer_for(object, serializer_for_options)
+      end
+
+      private
+
+      def cached_result
+        @cached_result ||= {}
+      end
+
+      def serialize_object!(object)
+        if collection?
+          if (serializer = instantiate_collection_serializer(object)).nil?
+            # BUG: per #2027, JSON API resource relationships are only id and type, and hence either
+            # *require* a serializer or we need to be a little clever about figuring out the id/type.
+            # In either case, returning the raw virtual value will almost always be incorrect.
+            #
+            # Should be reflection_options[:virtual_value] or adapter needs to figure out what to do
+            # with an object that is non-nil and has no defined serializer.
+            cached_result[:virtual_value] = object.try(:as_json) || object
+          else
+            cached_result[:serializer] = serializer
+          end
+        else
+          cached_result[:serializer] = instantiate_serializer(object)
+        end
+      end
+
+      # NOTE(BF): This serializer throw/catch should only happen when the serializer is a collection
+      # serializer.  This is a good reason for the reflection to have a to_many? type method.
+      def instantiate_serializer(object)
+        serializer_options = association_options.fetch(:parent_serializer_options).except(:serializer)
+        serializer_options[:serializer_context_class] = association_options.fetch(:parent_serializer).class
+        serializer = reflection_options.fetch(:serializer, nil)
+        serializer_options[:serializer] = serializer if serializer
+        serializer_class.new(object, serializer_options)
+      end
+
+      def instantiate_collection_serializer(object)
+        serializer = catch(:no_serializer) do
+          instantiate_serializer(object)
+        end
+        serializer
+      end
+
+      def namespace
+        reflection_options[:namespace] ||
+          association_options.fetch(:parent_serializer_options)[:namespace]
       end
     end
   end