Check abilities - Avoid common mistakes

You now know that you can use the can? method in the controller or view to check the user's permission for a given action and object.

can? :destroy, @article

and also that the cannot? method is for convenience and performs the opposite check of can?

cannot? :destroy, @article

What we want to explain you in this chapter is that you can also pass the class instead of a single instance:

<% if can? :create, Project %>
  <%= link_to "New Project", new_project_path %>
<% end %>

It's important to note here that if a block or hash of conditions exist they will be ignored when checking on a class, and it will return true. For example:

can :read, Project, priority: 3
can? :read, Project # returns true

It is impossible to answer this can? question completely because not enough detail is given. Here the class does not have a priority attribute to check on.

Think of it as asking

can the current user read a project?"

The user can read a project, so this returns true. However it depends on which specific project you're talking about.

If you are doing a class check, it is important you do another check once an instance becomes available so the hash of conditions can be used.

The reason for this behavior is because of the controller index action. Since the authorize_resource before filter has no instance to check on, it will use the Project class. If the authorization failed at that point then it would be impossible to filter the results later when Fetching Records.

That is why passing a class to can? will return true.

The code answering the question "can the user update all the articles?" would be something like:

Article.accessible_by(current_ability).count == Article.count