Rails has_many和belongs_to关联方法具有异常行为

Question

I recently stumbled upon a strange behaviour with some of the methods added by belongs_to and has_many .

Consider the following scenario, as presented by Rail's Active Record Association guide .

class Author < ApplicationRecord
  has_many :books, inverse_of: :author
end

class Book < ApplicationRecord
  belongs_to :author, inverse_of: :books
end

If I use belongs_to 's association=(associate) method , I loose reference to book from author:

book = Book.new
author = Author.new
book.author = author
author.books.include?(book) # false, expected true
author.books.empty? # true, expected to contain book

But if I make the association with has_many 's collection<<(object) method , the reference persists as expected:

book = Book.new
author = Author.new
author.books << book
author.books.include?(book) # true, as expected
author.books.empty? # false, as expected
book.author == author # true, as expected

Is this the expected behaviour? I don't quite understand why the first case does not store the association from author to book .

I'm using ruby 2.5.1 and rails 5.2.0.

Thanks in advance.

Answer 1

author.books << book builds an ActiveRecord::Associations::CollectionProxy . So with author.books.include? book author.books.include? book you are checking if it exists inside that collection (not yet stored in db): it's true.

With just book.save you can save both book and author .

Conversely, book.author = author is just an assignment. You need to author.save and book.save to effectively build the relation. If you check author.books.include?(book) after the two save, if becomes true.

Try it in rails console.