Rails: if has_many relationship changed

Question

I\'ve got these two classes.

class Article < ActiveRecord::Base
  attr_accessible :body, :issue, :name, :page, :image, :video, :brand_ids
  has_many :pub         


        

Answer 1

A bit late, but for other people who are looking for a similar solution, you can detect changes in the Relationship (also has_and_belongs_to_many) on this way:

class Article < ActiveRecord::Base
  attr_accessible :body, :issue, :name, :page, :image, :video, :brand_ids
  has_many :publications
  has_many :docs, :through => :publications
end

class Doc < ActiveRecord::Base
  attr_accessible :issue_id, :cover_id, :message, :article_ids, :user_id, :created_at, :updated_at, :issue_code, :title, :template_id
  has_many :publications, dependent: :destroy
  has_many :articles, :through => :publications, :order => 'publications.position'
  has_many :edits, dependent: :destroy
  accepts_nested_attributes_for :articles, allow_destroy: false

  after_initialize :initialize_article_changes_safe

  after_save :change_articles
  after_save :initialize_article_changes

  before_add_for_articles << ->(method,owner,change) { owner.send(:on_add_article, change) }
  before_remove_for_articles << ->(method,owner,change) { owner.send(:on_remove_article, change) }

  def articles_changed?
    @article_changes[:removed].present? or @article_changes[:added].present?
  end

  private

  def on_add_article(article)
    initialize_article_changes_safe
    @article_changes[:added] << article.id
  end

  def on_remove_article(article)
    initialize_article_changes_safe
    @article_changes[:removed] << article.id
  end

  def initialize_article_changes
    @article_changes = {added: [], removed: []}
  end

  def initialize_article_changes_safe
    @article_changes = {added: [], removed: []} if @article_changes.nil?
  end

  def unchanged_article_ids
    self.article_ids - @article_changes[:added] - @article_changes[:removed]
  end

  def change_articles
    do_stuff if self.articles_changed?
    do_stuff_for_added_articles unless @article_changes[:added].nil?
    do_stuff_for_removed_articles unless @article_changes[:removed].nil?
  end
end

The two hooks before_add_for_NAME-OF-RELATION and before_remove_for_NAME-OF-RELATION are triggered when adding or removing a relation. The triggered functions (you can not link a function by name, you have to do it by lambda) add the ids of the added / removed relation items to the @articel_changes hash. When the model is saved, you can handle the objects by their ids in the change_articles function. After it, the @articel_changes hash will be cleared.