ruby-on-rails - has_many和:after_add/:before_add => callback for << and create methods

Question

我正在阅读Rails 3 Way的书，并感到困惑:

:after_add => callback Called after a record is added to the collection via the << method. Is not triggered by the collection’s create method

据我了解book.chapters.create(title:'第一章')不会调用before_add回调，但实际上它正在调用。

class Book < ActiveRecord::Base
  attr_accessible :title
  has_many :chapters, :before_add => :add_chapter

  private
    def add_chapter(chapter)
      logger.error('chapter added to book')
    end
end

class Chapter < ActiveRecord::Base
  belongs_to :book
  attr_accessible :title
end

在控制台中(最小)

 > b = Book.first
  Book Load (0.1ms)  SELECT "books".* FROM "books" LIMIT 1
 > b.chapters.create(title: 'Last Chapter')
  begin transaction
chapter added to book
  INSERT INTO "chapters" ....
  commit transaction

在这里，您可以看到 after_add回调是为 create调用的。

我误会了吗？

编辑

b.chapters.new(title: 'New Chapter')
b.chapters.build(title: 'New Chapter')

也调用回调

Answer 1

将项目添加到集合时，将触发before_add和after_add回调。与是否将记录保存到数据库无关。 (对于has_and_belongs_to_many和has_many :through关系，这里会有一点异常(exception)，其中将其添加到集合中会立即由ActiveRecord在内部反射(reflect)到数据库中)。

将新记录添加到集合后，将触发回调。在将元素添加到集合之前，将调用before_add，在之后将after_add调用。

下面的示例可以使您更好地理解。

# id: integer
class Author < ActiveRecord::Base
  has_many :books, before_add: :bef, after_add: aft

  def bef
    puts "Before adding, author ##{id} has #{books.size} books"
  end

  def aft
    puts "After adding, author ##{id} has #{books.size} books"
  end
end

# id integer
# author_id: integer
class Book < ActiveRecord::Base
  belongs_to :author
  after_save: :saved

  def saved
    puts "The book is now saved!"
  end
end

> book = Book.new

> author = Author.first

> author.books << book
'Before adding, author #1 has 3 books'
'After adding, author #1 has 4 books'

> author.save
'The book is now saved'