define_validations

# File activerecord/lib/active_record/associations/builder/belongs_to.rb, line 112
    def self.define_validations(model, reflection)
      if reflection.options.key?(:required)
        reflection.options[:optional] = !reflection.options.delete(:required)
      end

      if reflection.options[:optional].nil?
        required = model.belongs_to_required_by_default
      else
        required = !reflection.options[:optional]
      end

      super

      if required
        if ActiveRecord.belongs_to_required_validates_foreign_key
          model.validates_presence_of reflection.name, message: :required
        else
          condition = lambda { |record|
            foreign_key = reflection.foreign_key
            foreign_type = reflection.foreign_type

            record.read_attribute(foreign_key).nil? ||
              record.attribute_changed?(foreign_key) ||
              (reflection.polymorphic? && (record.read_attribute(foreign_type).nil? || record.attribute_changed?(foreign_type)))
          }

          model.validates_presence_of reflection.name, message: :required, if: condition
        end
      end
    end

    def self.define_change_tracking_methods(model, reflection)
      model.generated_association_methods.class_eval         def #{reflection.name}_changed?          association(:#{reflection.name}).target_changed?        end        def #{reflection.name}_previously_changed?          association(:#{reflection.name}).target_previously_changed?        end, __FILE__, __LINE__ + 1
    end

    private_class_method :macro, :valid_options, :valid_dependent_options, :define_callbacks,
      :define_validations, :define_change_tracking_methods, :add_counter_cache_callbacks,
      :add_touch_callbacks, :add_default_callbacks, :add_destroy_callbacks
  end