fab-manager/app/models/event_price_category.rb

class EventPriceCategory < ActiveRecord::Base
  belongs_to :event
  belongs_to :price_category

  has_many :tickets

  validates :price_category_id, presence: true
  validates :amount, presence: true

  before_destroy :verify_no_associated_tickets

  protected
  def verify_no_associated_tickets
    tickets.count == 0
  end

end