admin.rb

# Let's assume these models Department, Subject, User, Allotment, Test
# Allotment table has department_id, subject_id, user_id
# Test table has department_id, :subject_id

# Problem: allow the user to edit a test if he is alloted the subject
# for that department OR if he is special (dean) OR if the number of allotments
# of that user in that department exceeds 3
#

class Ability
  include CanCan::Ability

  def initialize(user)
    can :edit, Test do |test|
      department = test.department
      user_allotments = department.allotments.select {|a| a.user_id == user.id}
      department.deans.include?(user) || user_allotments.size > 3 ||
        user_allotments.select {|a| a.subject_id == test.subject_id}.present?
    end
  end
end