Sideload root only

lib/active_model/serializer.rb

@@ -70,6 +70,7 @@ def to_s
     class_attribute :_embed
     self._embed = :objects
     class_attribute :_root_embed
+    class_attribute :_root_only
 
     class_attribute :cache
     class_attribute :perform_caching
@@ -249,6 +250,7 @@ def model_class
       def embed(type, options={})
         self._embed = type
         self._root_embed = true if options[:include]
+        self._root_only = true if options[:root_only]
       end
 
       # Defines the root used on serialization. If false, disables the root.
@@ -379,21 +381,40 @@ def include!(name, options={})
 
       options = default_embed_options.merge!(options)
       options[:value] ||= send(name)
-      association = association_class.new(name, options, self.options)
+
+      serializer_options = self.options.dup
+
+      if include_root_associations_only?
+        serializer_options[:include_root_only] = !root_serializer?
+      end
+
+      association = association_class.new(name, options, serializer_options)
 
       if association.embed_ids?
         node[association.key] = association.serialize_ids
 
         if association.embed_in_root? && hash.nil?
           raise IncludeError.new(self.class, association.name)
         elsif association.embed_in_root? && association.embeddable?
-          merge_association hash, association.root, association.serializables, unique_values
+          if include_root_associations_only? && root_serializer?
+            merge_association hash, association.root, association.serializables, unique_values
+          elsif !include_root_associations_only?
+            merge_association hash, association.root, association.serializables, unique_values
+          end
         end
       elsif association.embed_objects?
         node[association.key] = association.serialize
       end
     end
 
+    def include_root_associations_only?
+      options.key? :include_root_only
+    end
+
+    def root_serializer?
+      options[:include_root_only] == true
+    end
+
     # In some cases, an Array of associations is built by merging the associated
     # content for all of the children. For instance, if a Post has_many comments,
     # which has_many tags, the top-level :tags key will contain the merged list

test/serializer_test.rb

@@ -1518,4 +1518,52 @@ def test_only_option_takes_precedence_over_custom_defined_include_methods
       }
     }, post_serializer.as_json)
   end
+
+  def test_sideloading_can_be_configured_on_only_the_root_object
+    adam = User.new name: 'Adam Hawkins', id: 1
+
+    post1 = Post.new title: "Post 1", id: 1
+    post2 = Post.new title: "Post 2", id: 2
+
+    post1.author = adam 
+    post2.author = adam
+
+    post_serializer_class = Class.new(ActiveModel::Serializer) do
+      embed :id, include: true
+      has_one :author
+      attributes :id, :title
+    end
+
+    user_serializer_class = Class.new(ActiveModel::Serializer) do
+      embed :id, include: true
+      has_many :posts
+      attributes :id, :name
+    end
+
+    adam.class_eval do
+      define_method :active_model_serializer do
+        user_serializer_class
+      end
+
+      define_method :posts do
+        [post1, post2]
+      end
+    end
+
+    [post1, post2].each do |user|
+      user.class_eval do
+        define_method :active_model_serializer do
+          post_serializer_class
+        end
+      end
+    end
+
+    post_serializer = post_serializer_class.new post1, root: :post, include_root_only: true
+    hash = post_serializer.as_json
+
+    assert_equal({
+      post: { id: 1, title: 'Post 1', author_id: 1 },
+      authors: [{ id: 1, name: 'Adam Hawkins', post_ids: [1,2] }]
+    }, hash)
+  end
 end