Add include_data :if_sideloaded (#1931)

For JSONAPI, `include_data` currently means, "should we populate the
'data'" key for this relationship. Current options are true/false.

This adds the `:if_sideloaded` option. This means "only
populate the 'data' key when we are sideloading this relationship." This
is because 'data' is often only relevant to sideloading, and causes a
database hit.

Addresses https://github.com/rails-api/active_model_serializers/issues/1555

lib/active_model/serializer/concerns/associations.rb

@@ -83,7 +83,8 @@ module ActiveModel
       #   +default_include_directive+ config value when not provided)
       # @return [Enumerator<Association>]
       #
-      def associations(include_directive = ActiveModelSerializers.default_include_directive)
+      def associations(include_directive = ActiveModelSerializers.default_include_directive, include_slice = nil)
+        include_slice ||= include_directive
         return unless object
 
         Enumerator.new do |y|
@@ -91,7 +92,8 @@ module ActiveModel
             next if reflection.excluded?(self)
             key = reflection.options.fetch(:key, reflection.name)
             next unless include_directive.key?(key)
-            y.yield reflection.build_association(self, instance_options)
+
+            y.yield reflection.build_association(self, instance_options, include_slice)
           end
         end
       end

lib/active_model/serializer/concerns/configuration.rb

@@ -30,6 +30,7 @@ module ActiveModel
         # Make JSON API top-level jsonapi member opt-in
         # ref: http://jsonapi.org/format/#document-top-level
         config.jsonapi_include_toplevel_object = false
+        config.include_data_default = true
 
         config.schema_path = 'test/support/schemas'
       end

lib/active_model/serializer/reflection.rb

@@ -37,7 +37,7 @@ module ActiveModel
       def initialize(*)
         super
         @_links = {}
-        @_include_data = true
+        @_include_data = Serializer.config.include_data_default
         @_meta = nil
       end
 
@@ -69,17 +69,15 @@ module ActiveModel
       #       Blog.find(object.blog_id)
       #     end
       #   end
-      def value(serializer)
+      def value(serializer, include_slice)
         @object = serializer.object
         @scope = serializer.scope
 
-        if block
-          block_value = instance_exec(serializer, &block)
-          if block_value != :nil
-            block_value
-          elsif @_include_data
-            serializer.read_attribute_for_serialization(name)
-          end
+        block_value = instance_exec(serializer, &block) if block
+        return unless include_data?(include_slice)
+
+        if block && block_value != :nil
+          block_value
         else
           serializer.read_attribute_for_serialization(name)
         end
@@ -106,11 +104,11 @@ module ActiveModel
       #
       # @api private
       #
-      def build_association(parent_serializer, parent_serializer_options)
-        association_value = value(parent_serializer)
+      def build_association(parent_serializer, parent_serializer_options, include_slice = {})
+        association_value = value(parent_serializer, include_slice)
         reflection_options = options.dup
         serializer_class = parent_serializer.class.serializer_for(association_value, reflection_options)
-        reflection_options[:include_data] = @_include_data
+        reflection_options[:include_data] = include_data?(include_slice)
         reflection_options[:links] = @_links
         reflection_options[:meta] = @_meta
 
@@ -137,6 +135,14 @@ module ActiveModel
 
       private
 
+      def include_data?(include_slice)
+        if @_include_data == :if_sideloaded
+          include_slice.key?(name)
+        else
+          @_include_data
+        end
+      end
+
       def serializer_options(parent_serializer, parent_serializer_options, reflection_options)
         serializer = reflection_options.fetch(:serializer, nil)