ability.rb

class Ability
  include CanCan::Ability
  
  def initialize(user)
    user ||= User.new # guest user

    can :read, Band
    can :create, Band do |band|
      user.persisted?
    end
    can [:update, :destroy], Band do |band|
      band.user == user.id
    end
    
    can :read, [Band, Song]
    can :create, Song do |song|
      song.band.user == user.id
    end
    can [:update, :destroy], Song do |song|
      song.user == user
    end
  end
end