inverse_of

Active Record 提供了 :inverse_of 选项，可以通过它明确声明双向关联:

    class Author < ApplicationRecord
      has_many :books, inverse_of: 'writer'

end

    class Book < ApplicationRecord
      belongs_to :writer, class_name: 'Author', foreign_key: 'author_id'

end

在 has_many 声明中指定 :inverse_of 选项后，Active Record 便能识别双向关联:

    a = Author.first
    b = a.books.first
    a.first_name == b.writer.first_name # => true
    a.first_name = 'David'
    a.first_name == b.writer.first_name # => true