Active Record callbacks with has_many association - ruby-on-rails-4

Question

I want to run a before_save or after_add callback every time I add a child object to a parent object (has_many association). In the callback I want to set the end_date property on the parent (cohort) based on the end_date properties of all the children (courses).
class Cohort < ActiveRecord::Base
has_many :courses
before_save :update_end_date
def update_end_date
self.end_date = courses.order(:end_date).last.try(:end_date)
end
end
The problem I'm experiencing is that the courses are not yet persisted to the database in the before_save callback, so courses.order(:end_date) does not return the newly added course(s).
There are several workarounds I could use (e.g. using the Ruby courses.sort_by method or using after_save with update), but my impression is that using the Active Record order method, if possible, would be preferable in terms of efficiency and best practice. Is there a way to do this with Active Record in before_save, or what might be best practice for this? It seems like something that would come up a lot, but I'm having trouble finding solutions that work for me, so I feel like I must be thinking about it wrong. Thanks!

Answer 1

You could do an after save on the courses that may update the cohort if their end date is later than the cohorts end date. and an after destroy on the course, that tells the cohort to update its end date to correspond to the remaining courses.
class Course < ActiveRecord::Base
belongs_to :cohort
after_save :maybe_update_cohort_end_date
after_destroy :update_cohort_end_date
def maybe_update_cohort_end_date
if cohort && self.end_date > cohort.end_date
cohort.end_date = self.end_date
cohort.save
end
end
def update_cohort_end_date
cohort.update_end_date if cohort
end
end
class Cohort < ActiveRecord::Base
has_many :courses
def update_end_date
self.end_date = courses.order(:end_date).last.try(:end_date)
end
end
This way you only make an update if the new or updated course's end date would change the the cohorts end date. But also catches if a course is removed then check what the end date should be