Rails force to_param to return something even when not persisted

Question

I need to handle a particular case of generating email views with URLs constructed from non-persisted data.

Example : assume my user can create posts, and that triggers a post creation notification email, I'd like to send the user an example of fake post creation. For this, I am using a FactoryGirl.build(:post) and passing this to my PostMailer.notify_of_creation(@post)

In everyday Rails life, we use the route url_helpers by passing as argument the model itself, and the route generator will automatically convert the model into its ID to be used for the route URL generation (in article_path(@article), the routes helper converts @article into @article.id for constructing the /articles/:id URL.

I believe it is the same in ActiveRecord, but anyways in Mongoid, this conversion fails if the model is not persisted (and this is somewhat nice as it prevents the generation of URLs that may not correspond to actual data)

So in my specific case, URL generation crashes as the model is not persisted:

<%= post_url(@post_not_persisted) %> 

crashes with

ActionView::Template::Error: No route matches {:action=>"show", :controller=>"posts", :post_id=>#<Post _id: 59b3ea2aaba9cf202d4eecb6 ...

Is there a way I can bypass this limitation only in a very specific scope ? Otherwise I could replace all my resource_path(@model) by resource_path(@model.id.to_s) or better @model.class.name but this doesn't feel like the right situation...

EDIT :

The main problem is

Foo.new.to_param # => nil
# whereas
Foo.new.id.to_s # => "59b528e8aba9cf74ce5d06c0"

I need to force to_param to return the ID (or something else) even if the model is not persisted. Right now I'm looking at refinements to see if I can use a scoped monkeypatch but if you have better ideas please be my guest :-)

module ForceToParamToUseIdRefinement
  refine Foo do 
    def to_param
      self.class.name + 'ID'
    end
  end
end

However I seem to have a small scope problem when using my refinement, as this doesn't bubble up as expected to url_helpers. It works fine when using te refinement in the console though (Foo.new.to_param # => 59b528e8aba9cf74ce5d06c0)

Answer 1

I found a way using dynamic method override. I don't really like it but it gets the job done. I am basically monkeypatching the instances I use during my tests.

To make it easier, I have created a class method example_model_accessor that basically behaves like attr_accessor excepts that the setter patches the #to_param method of the object

def example_model_accessor(model_name)
  attr_reader model_name

  define_method(:"#{model_name}=") do |instance|
    def instance.to_param
      self.class.name + 'ID'
    end
    instance_variable_set(:"@#{model_name}", instance)
  end
end

Then in my code I can just use

class Testing
  example_model_accessor :message

  def generate_view_with_unpersisted_data
    self.message = FactoryGirl.build(:message)
    MessageMailer.created(message).deliver_now
  end
end

# views/message_mailer/created.html.erb
...
<%= message_path(@message) %> <!-- Will work now and output "/messages/MessageID" ! -->