Embed objects should return singular and not wrap in array

Closes #437

lib/active_model/serializer.rb

@@ -131,7 +131,8 @@ end
           if association.embed_ids?
             hash[association.key] = serialize_ids association
           elsif association.embed_objects?
-            hash[association.embedded_key] = serialize association
+            associated_data = send(association.name)
+            hash[association.embedded_key] = serialize(association, associated_data)
           end
         end
       end
@@ -147,15 +148,15 @@ end
       associations.each_with_object({}) do |(name, association), hash|
         if included_associations.include? name
           if association.embed_in_root?
-            hash[association.embedded_key] = serialize association
+            associated_data = Array(send(association.name))
+            hash[association.root_key] = serialize(association, associated_data)
           end
         end
       end
     end
 
-    def serialize(association)
-      associated_data = send(association.name)
-      association.build_serializer(associated_data).serializable_object
+    def serialize(association, object)
+      association.build_serializer(object).serializable_object
     end
 
     def serialize_ids(association)

lib/active_model/serializer/associations.rb

@@ -17,24 +17,22 @@ module ActiveModel
         @embed_in_root = options.fetch(:embed_in_root) { options.fetch(:include) { CONFIG.embed_in_root } }
         @embed_key     = options[:embed_key] || :id
         @key           = options[:key]
+        @embedded_key  = options[:root] || name
 
         self.serializer_class = @options[:serializer]
       end
 
       attr_reader :name, :embed_ids, :embed_objects, :serializer_class
-      attr_accessor :embed_in_root, :embed_key, :key, :embedded_key, :options
+      attr_accessor :embed_in_root, :embed_key, :key, :embedded_key, :root_key, :options
       alias embed_ids? embed_ids
       alias embed_objects? embed_objects
       alias embed_in_root? embed_in_root
 
       def serializer_class=(serializer)
         @serializer_class = serializer.is_a?(String) ? serializer.constantize : serializer
-
         if @serializer_class && !(@serializer_class <= ArraySerializer)
           @options.merge!(each_serializer: @serializer_class)
           @serializer_class = ArraySerializer
-        else
-          @serializer_class ||= ArraySerializer
         end
       end
 
@@ -43,24 +41,30 @@ module ActiveModel
         @embed_objects = embed == :object || embed == :objects
       end
 
-      def build_serializer(object)
-        @serializer_class.new(Array(object), @options)
-      end
-
       class HasOne < Association
         def initialize(name, *args)
           super
-          @embedded_key = "#{@options[:root] || name}".pluralize
+          @root_key = @embedded_key.to_s.pluralize
           @key ||= "#{name}_id"
         end
+
+        def build_serializer(object)
+          @serializer_class ||= Serializer.serializer_for(object) || DefaultSerializer
+          @serializer_class.new(object, @options)
+        end
       end
 
       class HasMany < Association
         def initialize(name, *args)
           super
-          @embedded_key = @options[:root] || name
+          @root_key = @embedded_key
           @key ||= "#{name.to_s.singularize}_ids"
         end
+
+        def build_serializer(object)
+          @serializer_class ||= ArraySerializer
+          @serializer_class.new(object, @options)
+        end
       end
     end
   end