Run before_save after changed in attribute in a related model

I am having some troubles today and cannot think outside the box to fix this one.

Basically I have a model called Airplane, which has_many Payments. Each payment can be divided in many Installments. Ok!

Here is the info:

Model Airplane
- has_many payments
- before_save :checks_if_everything_has_been_paid

Model Payment
- belongs_to airplane
- has_many installments

Model Installment
- belongs_to payment

So, what I want to do is when the sum of the installments be equal or greater than the Airplane ticket value then the Airplane.paid will be true. I am doing that using the before_save "checks_if_everything_has_been_paid. But it only works when there are changes on the Airplane fields.

How can I run this class when there are changes both in the Payment and Installment fields?

I want to check if the payment is completed everytime an installment is changed or the Payment itself.

Thank you!

Solution

Instead of defining an after save callback on the Airplane model, define a after_add callback on the payments association.

class Airplane < ActiveRecord::Base
  has_many :payments, after_add: :checks_if_everything_has_been_paid

  def checks_if_everything_has_been_paid
    # work some magic
  end
end

Update: I think the following may be a better approach if I understand your data model correctly. If a payment or installment is saved it will trigger the airplane to check for full payment:

class Airplane < ActiveRecord::Base
  has_many :payments
  has_many :installments, through: :payments

  def check_for_full_payment
    # work some magic
  end
end

class Payment < ActiveRecord::Base
  belongs_to :airplane
  has_many :installments

  after_save :trigger_full_payment_check

  def trigger_payments_check
    airplane.check_for_full_payment
  end
end

class Installment < ActiveRecord::Base
  belongs_to :payment

  delegate :airplane, to: :payment

  after_save :trigger_full_payment_check

  def trigger_payments_check
    airplane.check_for_full_payment
  end
end

The nice thing about this approach is that the logic in Payment and Installment is identical, so you can extract it to a module:

module TriggerFullPaymentCheck
  def self.included(base)
    base.after_save :trigger_full_payment_check
  end

  def trigger_payments_check
    airplane.check_for_full_payment
  end
end

class Airplane < ActiveRecord::Base
  has_many :payments
  has_many :installments, through: :payments

  def check_for_full_payment
    # work some magic
  end
end

class Payment < ActiveRecord::Base
  include TriggerFullPaymentCheck

  belongs_to :airplane
  has_many :installments
end

class Installment < ActiveRecord::Base
  include TriggerFullPaymentCheck

  belongs_to :payment
  delegate :airplane, to: :payment
end