ability_with_callable_policy_object.rb

class AgencyVisitPolicy
  def initialize(user)
    @user = user
  end

  def to_proc
    Proc.new { |visit| visit.agency_id.in?([@user.agency_id, nil]) }
  end
end

class Ability
  include CanCan::Ability

  def initialize(user)
    if user.agency?
      can :manage, Visit do |visit|
        visit.agency_id.in?([user.agency_id, nil])
      end

      # this does the same:
      can :manage, Visit, &AgencyVisitPolicy.new(user)
    end
  end
end