Context aware authorization using CanCan

Question

I want to use CanCan to handle my permissions. My site has many different permissions levels, and most of them are context aware. For instance, Here are the relations in my 3 main models:

class User < ActiveRecord::Base
  has_many :league_relations
  has_many :leagues, :through => :league_relations
end

class League < ActiveRecord::Base
  has_many :league_relations
  has_many :users, :through => :league_relations
end

class LeagueRelation < ActiveRecord::Base
  belongs_to :user
  belongs_to :league
end

Note, LeagueRelations is a nested resource of Leagues. What I want to do is allow a user to modify leagues, and gauge each user's authorization based off of data stored in league_relation. I would then like a user to modify league relation, based only the data stored in the user model.

To be succinct: I basically want LeagueRelations to be used to authorize League actions, and Users to be used to authorize LeagueRelations actions. i.e. league_relation.owner = true to delete a League, but user.owner? must be true to delete a LeagueRelation. How can I authorize based on the attributes of league_relation when inside the league controller, and authorize other actions in other controllers on other models. Please leave a comment if you need more clarification.

Thanks.

Answer 1

Ok, I solved the problem. My use case is briefly mentioned in the beginning of the CanCan README and I missed it. You can define new Ability classes in app/models/ that take in a different parameter other than current_user. To do so, you put the following in your controller:

def current_ability 
  if params[:controller] == 'leagues'
    @current_ability = LeagueAbility.new(current_user_league_relation)
  elsif params[:controller] == 'league_relations'
    @current_ability = LeagueRelationAbility.new(current_user_league_relation)
  else
    @current_ability = Ability.new(current_user)
  end
end

Now you can create league_ability.rb in app/models/.

class LeagueAbility
  include CanCan::Ability

  def initialize(league_relation)
    league_relation ||= LeagueRelation.new

    if league_relation.owner?
      can :manage, League, :id => league_relation.league_id
    elsif league_relation.moderator?
      can :manage, League, :id => league_relation.league_id
      cannot [:delete, :destroy], League
    else
      can :read, League
      can :create, League
    end    
  end
end

One thing to note is that this relies on your application controller calling a method in a child class. Hope that helps!