Finish convert_to_object updates

actionpack/lib/action_view/helpers/form_helper.rb

@@ -1029,7 +1029,7 @@ def nested_attributes_association?(association_name)
         def fields_for_with_nested_attributes(association_name, args, block)
           name = "#{object_name}[#{association_name}_attributes]"
           association = @object.send(association_name)
-          explicit_object = args.first if args.first.respond_to?(:new_record?)
+          explicit_object = args.first.to_model if args.first.respond_to?(:to_model)
 
           if association.is_a?(Array)
             children = explicit_object ? [explicit_object] : association
@@ -1044,6 +1044,8 @@ def fields_for_with_nested_attributes(association_name, args, block)
         end
 
         def fields_for_nested_model(name, object, args, block)
+          object = convert_to_model(object)
+
           if object.new_record?
             @template.fields_for(name, object, *args, &block)
           else

actionpack/test/template/form_helper_test.rb

@@ -43,6 +43,8 @@ def relevances_attributes=(attributes); end
   end
 
   class Tag
+    extend ActiveModel::APICompliant
+
     attr_reader :id
     attr_reader :post_id
     def initialize(id = nil, post_id = nil); @id, @post_id = id, post_id end
@@ -59,6 +61,8 @@ def relevances_attributes=(attributes); end
   end
 
   class CommentRelevance
+    extend ActiveModel::APICompliant
+
     attr_reader :id
     attr_reader :comment_id
     def initialize(id = nil, comment_id = nil); @id, @comment_id = id, comment_id end
@@ -71,6 +75,8 @@ def value
   end
 
   class TagRelevance
+    extend ActiveModel::APICompliant
+
     attr_reader :id
     attr_reader :tag_id
     def initialize(id = nil, tag_id = nil); @id, @tag_id = id, tag_id end