Unfortunately, this is a bug. You’ll have to set the url like you mention.
= form_for @order, :url => order_path do |f|
Note that this will properly route to create or update depending on whether @order is persisted.
Update
There’s another option now. You can add this to your routes config:
resolve("Order") { [:order] }
Then when the polymorphic_url method is given an object with class name “Order” it will use [:order] as the url component instead of calling model_name.route_key as described in jskol’s answer.
This has the limitation that it cannot be used within scopes or namespaces. You can route a namespaced model at the top level of the routes config:
resolve('Shop::Order') { [:shop, :order] }
But it won’t have an effect on routes with extra components, so
url_for(@order) # resolves to shop_order_url(@order)
url_for([:admin, @order]) # resolves to admin_shop_orders_url(@order)
# note plural 'orders' ↑
# also, 'shop' is from module name, not `resolve` in the second case