Model Association Before and After Update - ruby-on-rails

Question

Say I have a Rails model and I want to get the state of a list of an association before and after an update to see the difference between them. How would I go about doing this? I had tried just doing a before_update and after_update callback to maintain an array of the association and taking a rejection of what was in the array after the update compared to before the update but it seems that my array is never matching the state before the update.
Any ideas?
# == Schema Information
#
# Table name: lectures
#
# id :integer not null, primary key
# organization_id :integer
# training_type_id :integer
# name :string(255)
# description :text
# start_date :date
# end_date :date
# training_method :string(255)
# trainer_name :string(255)
# trainer_phone :string(255)
# trainer_email :string(255)
# location :string(255)
# total_seats :integer
# available_seats :integer
# hours :integer
# created_at :datetime
# updated_at :datetime
#
class Lecture < ActiveRecord::Base
# # # # # # # # # # # # # # #
# Relations
# # # # # # # # # # # # # # #
belongs_to :organization
belongs_to :training_type
has_many :training_histories
has_many :users, through: :training_histories
# belongs_to :training_type
# has_many :training_histories, inverse_of: :lecture, :autosave => true
accepts_nested_attributes_for :training_histories
# # # # # # # # # # # # # # #
# Validations
# # # # # # # # # # # # # # #
validates :name,
:start_date,
:end_date,
:location,
:organization,
:trainer_email,
:trainer_name,
:hours,
:presence => :true
validate :start_must_be_before_end_date
validates :trainer_email, email: true
after_update :email_users_about_update, email_supervisors_about_signup
def start_must_be_before_end_date
errors.add(:start_date, "must be before end date") unless self.start_date <= self.end_date
end
def email_users_about_update
self.training_histories.each do |history|
UserMailer.updated_training_email(history.user, self).deliver unless history.completed
end
end
def email_supervisors_about_signup
binding.pry
if self.users.changed?
users_dif = self.users - self.users_was
unless users_dif.nil?
users_dif.each do |user|
UserMailer.create_subordinate_training_sign_up(user.leader, user, self).deliver
end
end
end
end
end

Answer 1

You can use an after_add callback:
# add the callback to our association
has_many :users, through: :training_histories, after_add: :invite_to_training
# this method gets called when a user is added
private
def invite_to_training(user)
UserMailer.create_subordinate_training_sign_up(user.leader, user, self).deliver
end
For more information on association callbacks see: http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#module-ActiveRecord::Associations::ClassMethods-label-Association+callbacks