Callbacks - Set field on model

Question

I am trying the following - I have Models: Tales, Books, Keywords

class Tale < ActiveRecord::Base
has_many :tale_culture_joins
has_many :cultures, through: :tale_culture_joins
has_many :tale_purpose_joins
has_many :purposes, through: :tale_purpose_joins
has_many :tale_book_joins
has_many :books, through: :tale_book_joins
has_many :tale_keyword_joins
has_many :keywords, through: :tale_keyword_joins

class Book < ActiveRecord::Base
has_many :tale_book_joins
has_many :tales, through: :tale_book_joins
end

class TaleBookJoin < ActiveRecord::Base
belongs_to :tale
belongs_to :book
end

class Keyword < ActiveRecord::Base
has_many :tale_keyword_joins
has_many :tales, through: :tale_keyword_joins
end

class TaleKeywordJoin < ActiveRecord::Base
belongs_to :tale
belongs_to :keyword
end

These are the migrations

class CreateTales < ActiveRecord::Migration
  def change
    create_table :tales do |t|
    t.text :name, null: false, unique: true
    t.boolean :exists, default: nil
    t.timestamps null: false
    end
  end
end

class CreateBooks < ActiveRecord::Migration
  def change
  create_table :books do |t|
    t.text :name, null: false, unique: true
    t.boolean :exists, default: nil
    t.timestamps null: false
  end
end
end

class CreateKeywords < ActiveRecord::Migration
  def change
    create_table :keywords do |t|
      t.text :name, null: false, unique: true
      t.boolean :exists, default: nil
      t.timestamps null: false
    end
  end
end

What I want to happen is that everytime i delete a join between (Tale, Book) or (Tale,Keyword) through following method tale_instance_object.book_ids = []

It should go and check if the books for whom the relations have been broken have any other tale relations. If not then set :exists in Book object instance to false.

I am able to do this through controller code. Wondering how CallBacks or ActiveModel can be used

Answer 1

Join classes should only really be used when the relation is an object on its own. Consider these cases:

doctors -> appointments <- patients
years -> days <- hours

In these cases the relation object has data of its own (appointments.time, days.weekday) and logic. Otherwise you are just wasting memory since a object has to instantiated for every relation. Use has_and_belongs_to instead.

class Tale < ActiveRecord::Base
  # has_and_belongs_to_many :cultures
  # has_and_belongs_to_many :purposes
  has_and_belongs_to_many :books
  has_and_belongs_to_many :keywords 
  after_destroy :update_books!
end

class Book
  has_and_belongs_to_many :tales

  def check_status!
    self.update_attribute(status: :inactive) unless books.any
  end
end

class Keyword
  has_and_belongs_to_many :tales
end 

Also exists is a really bad naming choice for a model field since it collides with Rails built in exists? methods. This will have unexpected consequences.

A better alternative would be to use an integer combined with an enum.

class Book
  # ...
  enum :status [:inactive, :active] # defaults to inactive
end

class CreateBooks < ActiveRecord::Migration
  def change
  create_table :books do |t|
    t.text :name, null: false, unique: true
    t.integer :status, default: 0
    t.timestamps null: false
  end
end

This will also add the methods book.inactive? and book.active?.

You could add a callback to Tale which tells Book to update when a tale is updated but this code really stinks since Tale now is responsible for maintaining state of Book.

class Tale < ActiveRecord::Base
  # ...
  after_destroy :update_books!

  def update_books!
    self.books.each { |b| b.check_status! }
  end
end

class Book
  has_and_belongs_to_many :tales

  def check_status!
    self.update_attribute(status: :inactive) unless tales.any?
  end
end

A better alternative to add a callback on Books or to do it in the controller when destroying a tale:

class Book
  has_and_belongs_to_many :tales

  before_save :check_for_tales!, if: -> { self.tales_changed? }

  def check_for_tales!
    self.status = :inactive unless self.tales.any?
  end
end