ability.rb

class Ability
  include CanCan::Ability

  def initialize(user)
    user ||= User.new # guest user

    # if a member, they can manage their own posts 
    # (or create new ones)
    if user.role? :member
      can :manage, Wiki, private: true, Wiki.collaborators do |collaborator|
        collaborator.user_id == user.id
      end
    end

    #...

  
  end
end