ruby-on-rails - rails - 左移 "<<"运算符自动保存记录

Question

需要帮助理解这段代码，据我所知，我知道“<<”附加到一个集合，但在这里它正确地保存了记录，为什么不调用 .save 方法就可以呢？

#user.rb
 has_many :saved_properties, through: :property_saves, source: :property

#users_controller.rb
def update
  if @user.saved_properties << Property.find(params[:saved_property_id])
        render plain: "Property saved"
end

Answer 1

在 has_many 中 documentation它说:

Adds one or more objects to the collection by setting their foreign keys to the collection's primary key. Note that this operation instantly fires update SQL without waiting for the save or update call on the parent object, unless the parent object is a new record.