sync with upstream

lib/active_model/serializer/association/has_many.rb

@@ -0,0 +1,36 @@
+module ActiveModel
+  class Serializer
+    class Association
+      class HasMany < Association
+        def initialize(name, *args)
+          super
+          @root_key = @embedded_key
+          @key ||= "#{name.to_s.singularize}_ids"
+        end
+
+        def serializer_class(object, _)
+          if use_array_serializer?
+            ArraySerializer
+          else
+            serializer_from_options
+          end
+        end
+
+        def options
+          if use_array_serializer?
+            { each_serializer: serializer_from_options }.merge! super
+          else
+            super
+          end
+        end
+
+        private
+
+        def use_array_serializer?
+          !serializer_from_options ||
+            serializer_from_options && !(serializer_from_options <= ArraySerializer)
+        end
+      end
+    end
+  end
+end