nested_resources.md

Nested Resources

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

resources :projects do
  resources :tasks
end

We can then tell CanCanCan 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, but calling just authorize_resource for the parent object is insufficient. The task is then loaded through the @project.tasks association.

If the name of the association doesn't match the resource name, for instance has_many :issues, class_name: 'Task', you can specify the association name using :through_association.

  class TasksController < ApplicationController
    load_and_authorize_resource :project
    load_and_authorize_resource :task, through: :project, through_association: :issues
  end

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.

Securing through changes

If you are using through, you need to be wary of potential changes to the parent model. For example, consider this controller:

class TasksController < ApplicationController
  load_and_authorize_resource :project
  load_and_authorize_resource :task, through: :project

  def update
    @task.update(task_params)
  end

  private

  def task_params
    params.require(:task).permit(:project_id)
  end
end

Now consider a request to /projects/1/tasks/42 with params { task: { project_id: 2 } }.

• load_and_authorize_resource :project will load project 1 and authorize it.
• load_and_authorize_resource :task, through: :project will load task 42 from project 1, and authorize it.
• @task.update(task_params) will change the task's project ID from 1, to 2.
• Project 2 is never authorized! An attacker could inject a project belonging to another customer here.

How you handle this depends on your intended behavior.

• If you don't want a task's project ID to ever change, don't permit it as a param.
• If you allow tasks to be moved between projects, manually verify the ID change and avoid mass assigning it.

  def update
    @task.project = Project.find(task_params[:project_id])
    authorize!(@task)
    @task.assign(task_params.except(:project_id))
  end

Nested through method

It's also possible to nest through a method, this is commonly the current_user method.

class ProjectsController < ApplicationController
  load_and_authorize_resource through: :current_user
end

Here everything will be loaded through the current_user.projects association.

Shallow nesting

The parent resource is required to be present and it will raise an exception if the parent is ever nil. If you want it to be optional (such as with shallow routes), add the shallow: true option to the child.

class TasksController < ApplicationController
  load_and_authorize_resource :project
  load_and_authorize_resource :task, through: :project, shallow: true
end

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_resource :project
load_resource :event
load_and_authorize_resource :task, through: [:project, :event]

Here it will check both the @project and @event variables and fetch the task through whichever one exists. Note that this is only loading the parent model, if you want to authorize the parent you will need to do it through a before_action because there is special logic involved.

before_action :authorize_parent

private

def authorize_parent
  authorize! :read, (@event || @project)
end

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.

can? :create, @project.tasks.build

It's also possible to check permission through an association like this.

can? :read, @project => Task

This will use the above :project hash conditions and ensure @project meets those conditions.

Has_many through associations

How to load and authorize resources with a has_many :through association?

Given that situation:

class User < ActiveRecord::Base
  has_many :groups_users
  has_many :groups, through: :groups_users
end

class Group < ActiveRecord::Base
  has_many :groups_users
  has_many :users, through: :groups_users
end

class GroupsUsers < ActiveRecord::Base
  belongs_to :group, inverse_of: :groups_users
  belongs_to :user, inverse_of: :groups_users
end

and in the controller:

class UsersController < ApplicationController
  load_and_authorize_resource :group
  load_and_authorize_resource through: :group

in ability.rb

can :create, User, groups_users: { group: { CONDITION_ON_GROUP } }

Don't forget the inverse_of option, it is the trick to make it work correctly.

Remember to define the ability through the groups_users model (i.e. don't write can :create, User, groups: { CONDITION_ON_GROUP })

You will be able to persist the association just calling @user.save instead of @group.save.