Fix issue with embedding multiple associations under the same root key

lib/active_model/serializer.rb

@@ -204,7 +204,9 @@ end
           association_serializer = build_serializer(association)
           # we must do this always because even if the current association is not
           # embeded in root, it might have its own associations that are embeded in root
-          hash.merge!(association_serializer.embedded_in_root_associations) {|key, oldval, newval| [newval, oldval].flatten }
+          hash.merge!(association_serializer.embedded_in_root_associations) do |key, oldval, newval|
+            oldval.merge(newval) { |_, oldval, newval| [oldval, newval].flatten.uniq }
+          end
 
           if association.embed_in_root?
             if association.embed_in_root_key?

test/fixtures/active_record.rb

@@ -74,6 +74,10 @@ class ARSectionSerializer < ActiveModel::Serializer
   attributes 'name'
 end
 
+class AREmbeddedSerializer < ActiveModel::Serializer
+  has_many :ar_tags, :ar_comments
+end
+
 ARPost.create(title: 'New post',
               body:  'A body!!!',
               ar_section: ARSection.create(name: 'ruby')).tap do |post|

test/integration/active_record/active_record_test.rb

@@ -58,6 +58,22 @@ module ActiveModel
         end
       end
 
+      def test_serialization_embedding_ids_in_common_root_key
+        post_serializer = AREmbeddedSerializer.new(@post)
+
+        embed(AREmbeddedSerializer, embed: :ids, embed_in_root: true, embed_in_root_key: :linked) do
+          embed(ARCommentSerializer, embed: :ids, embed_in_root: true, embed_in_root_key: :linked) do
+            assert_equal({
+              'ar_tags' => [{ name: 'short' },
+                            { name: 'whiny' },
+                            { name: 'happy' }],
+              'ar_comments' => [{ body: 'what a dumb post', 'ar_tag_ids' => [3, 2] },
+                                { body: 'i liked it', 'ar_tag_ids' => [3, 1] }]
+            }, post_serializer.as_json[:linked])
+          end
+        end
+      end
+
       private
 
       def embed(serializer_class, options = {})
@@ -66,6 +82,7 @@ module ActiveModel
         serializer_class._associations.each_value do |association|
           association.embed = options[:embed]
           association.embed_in_root = options[:embed_in_root]
+          association.embed_in_root_key = options[:embed_in_root_key]
         end
 
         yield