Nested Resources

Let's say we have a nested resources set up in our routes.

resources :projects do
  resources :tasks
end

We can then tell CanCan to load the project and then load the task through that.

class TasksController < ApplicationController
  load_and_authorize_resource :project
  load_and_authorize_resource :task, :through => :project
end

This will fetch the project using Project.find(params[:project_id]) on every controller action, save it in the @project instance variable, and authorize it using the :read action to ensure the user has the ability to access that project. If you don't want to do the authorization you can simply use load_resource. The task is then loaded through the @project.tasks association.

If the resource name (:project in this case) does not match the controller then it will be considered a parent resource. You can manually specify parent/child resources using the :parent => false option.

Singleton Resource

What if each project only had one task through a has_one association? To set up singleton resources you can use the :singleton option.

class TasksController < ApplicationController
  load_and_authorize_resource :project
  load_and_authorize_resource :task, :through => :project, :singleton => true
end

It will then use the @project.task and @project.build_task methods for fetching and building respectively.

Polymorphic Associations

Let's say tasks can either be assigned to a Project or an Event through a polymorphic association. An array can be passed into the :through option and it will use the first one it finds.

load_and_authorize_resource :project
load_and_authorize_resource :event
load_and_authorize_resource :task, :through => [:project, :event]

Here it will check for the existence of the @project or @event variable and whichever one exists it will fetch the task through that.

Accessing Parent in Ability

Sometimes the child permissions are closely tied to the parent resource. For example, if there is a user_id column on Project, one may want to only allow access to tasks if the user owns their project.

This will happen automatically due to the @project instance being authorized in the nesting. However it's still a good idea to restrict the tasks separately. You can do so by going through the project association.

# in Ability
can :manage, Task, :project => { :user_id => user.id }

This means you will need to have a project tied to the tasks which you pass into here. For example, if you are checking if the user has permission to create a new task, do that by building it through the project.

<% if can? :create, @project.tasks.build %>

Basically, if you need to access the parent resource, be sure to pass it in with the child instance.