Cancan Ability during the creation of new objects

I have the following:

User has_many Listings
Listing has_many Offers

Pretty basic. A user can publish listings and other users can make offers on those listings.

On each Listing View, you can see a description of the listing, and also a list of all of the offers it has received. Also, on this same view, there is a form which lets users make an offer on this Listing.

Of course, if I am looking at my own Listing, I do not want to be able to make an Offer. So, I'm trying to add a restriction on the Offer's create method, to only allow access to it if it is from a User different from the one that posted this Listing. In my Ability class, it would be something like:

can :create, Offer if listing.user != user

This doesn't work since listing is not defined anywhere. This method will be called when a use开发者_运维百科r is viewing a certain listing, so, how can I pass this current Listing to the cancan method to effectively check this restriction?

Thanks.

---

Move the CanCan ability check until after the point at which you've created the new offer. E.g.

#listing.rb
load_and_authorize_resource :except => :create

def create
    @offer = #whatever it is you want to do
    authorize! :create, @offer

end