Authorizing polymorphic association with CanCan 1.6.9

ability.rb

class Ability
  include CanCan::Ability
  
  def initialize(user, session=nil)
    @user = user
    @session = session
    @user ? user_rules : public_rules
  end

  def user_rules
    can :update, Building, coach_id: @user.id
    can :manage, Assignment do |assignment|
      self.can? :update, assignment.assignable
    end
  end
end

assignment.rb

class Assignment < ActiveRecord::Base
  belongs_to :assignable, polymorphic: true
  belongs_to :user
end

assignments_controller.rb

class AssignmentsController < ApplicationController
  before_filter :load_assignments
  authorize_resource

  private

  def load_assignable
    if params[:program_id]
      @assignable = @program = Program.find(params[:program_id])
    elsif params[:building_id]
      @assignable = @building = Building.find(params[:building_id])
    elsif params[:classroom_id]
      @assignable = @classroom = Classroom.find(params[:classroom_id])
    end
  end

  def load_assignments
    load_assignable

    if @assignable
      @assignments = @assignable.assignments
    elsif params[:user_id]
      @user = User.find(params[:user_id])
      @assignments = @user.assignments
    else
      @assignments = Assignment.all.select {|a| can? :read, a}
    end
  end
end

building.rb

class Building < ActiveRecord::Base
  belongs_to :coach, class_name: "User"
  has_many :assignments, as: :assignable, dependent: :destroy
end

program.rb

class Program < ActiveRecord::Base
  has_many :assignments, as: :assignable, dependent: :destroy
  has_many :buildings
  belongs_to :coach, class_name: "User"
end