Ruby on rails: singular resource and form_for

I want user to work with only one order connected to user's session. So I set singular resource for order

routes.rb:

resource :order

views/orders/new.html.erb:

<%= form_for @order do |f| %>
   ...
<% end %>

But when I open the new order page I get an error:

undefined method `orders_path`

I know, that I can set :url => order_path in form_for, but what is the true way of resolving this collision?

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

Where does that magic path come from?

It took me a lot of tracing but I ultimately found that the url_for determines the path for your model using the polymorphic_path method defined in ActionDispatch::Routing::PolymorphicRoutes. polymorphic_path ultimately gets the automagic path for your model by calling something along the lines of:

record.class.model_name.route_key

I'm simplifying slightly but this is basically it. If you have an array (e.g. form_for[@order, @item]) the above is called on each element and the results are joined with _.

The model_name method on your Class comes from ActiveRecord::Naming.

module ActiveModel
  ...
  module Naming
    ...
    def model_name
      @_model_name ||= begin
        namespace = self.parents.detect do |n|
          n.respond_to?(:use_relative_model_naming?) && 
                                                 n.use_relative_model_naming?
        end
        ActiveModel::Name.new(self, namespace)
      end
    end
  end
end

How can I change it?

Fortunately ActiveModel::Name precalculates all values including route_key, so to override that value all we have to do is change the value of the instance variable.

For the :order resource in your question:

class Order < ActiveRecord::Base
  model_name.instance_variable_set(:@route_key, 'order')
  ...
end

# new.html.erb
<%= form_for @order do |f| # Works with action="/order" %>
    ...
<% end %>

Try it out!