Allow to set embed options from AM::Serializer

lib/active_model/serializer.rb

@@ -17,6 +17,11 @@ module ActiveModel
         yield SETTINGS
       end
 
+      def embed(type, options={})
+        SETTINGS[:embed] = type
+        SETTINGS[:include] = true if options[:include]
+      end
+
       def serializer_for(resource)
         if resource.respond_to?(:to_ary)
           ArraySerializer

lib/active_model/serializer/associations.rb

@@ -8,8 +8,8 @@ module ActiveModel
         @name          = name
         @options       = options
 
-        self.embed     = options[:embed]
-        @embed_in_root = @embed_ids && options[:include]
+        self.embed     = options[:embed] || SETTINGS[:embed]
+        @embed_in_root = @embed_ids && (options[:include] || SETTINGS[:include])
         @embed_key     = options[:embed_key] || :id
         @key           = options[:key]
         @embedded_key  = options[:root]

test/unit/active_model/serializer/has_many_test.rb

@@ -84,12 +84,16 @@ module ActiveModel
       end
 
       def test_associations_embedding_ids_including_objects_serialization_using_as_json
-        @association.embed_in_root = true
+        PostSerializer.embed :ids, include: true
+        PostSerializer._associations[0].send :initialize, @association.name, @association.options
+
         @post_serializer.root = nil
         assert_equal({
           'post' => { 'title' => 'Title 1', 'body' => 'Body 1', 'comment_ids' => @post.comments.map { |c| c.object_id } },
           'comments' => [{ 'content' => 'C1' }, { 'content' => 'C2' }]
         }, @post_serializer.as_json)
+      ensure
+        SETTINGS.clear
       end
 
       def test_associations_using_a_given_serializer