CanCan和能力阻挡

Question

I have this piece of code that defines CanCan's user permissions, but I don't understand what the conditions inside the block are for ?

    class Ability
          include CanCan::Ability

          def initialize(user)

            user.permissions.each do |permission|
              can permission.action.to_sym,
              permission.thing_type.constantize do |thing|
               thing.nil? || permission.thing_id.nil? || permission.thing_id == thing.id
              end
            end


end
end

So, this lines of code are bugging me:

permission.thing_type.constantize do |thing|
       thing.nil? || permission.thing_id.nil? || permission.thing_id == thing.id
end

I understand that block is used to define complex conditions and I guess that permission.thing_id == thing.id is there so that permission is granted only for selected objects but I don't see the purpoose of thing.nil? || permission.thing_id.nil? thing.nil? || permission.thing_id.nil? .What's that for?

Answer 1

So, according to the CanCan docs , when you pass a block to can (which is what's going on here), the permission is given only if the block returns true. The object passed in to the block is the object that the user might have permission to do something to.

What this does, then, is give the user permission to perform permission.action on the class permission.thing_type if:

1. thing is nil. This might happen in the case of things like create permission, which doesn't take a specific object (it takes the class instead).
2. permission.thing_id is nil. This is dealing with the case where the Permission object doesn't point at a specific thing. This feels like the opposite side of item 1. It might apply in the case of 'blanket' permissions - a permission that says a particular user can perform this action on any object of this class. OR
3. The thing has the id specified in the permission object.

Moral: Comment your code. Even if it's obvious to you right now what something does, it might not be to the next person. Who might also be you, in a few months.