如何级联保存与mongoid的has_and_belongs_to_many关系

Question

我有一个Tag和一个Question类。

class Question
  include Mongoid::Document
  include Mongoid::Timestamps
  identity type: String
  has_and_belongs_to_many :tags # Refer to tag_ids
end
class Tag
  include Mongoid::Document
  identity type: String    
  has_and_belongs_to_many :questions
end

然而，当我执行以下操作时，标签不会保存：

t = Tag.new(id: "politics")
q = Question.new({tags: [t]})
q.save!

然而，当我这样做时，标签是创建的：

Question.create!({tags: [Tag.new(id: "politics")]})

Answer 1

如果我执行以下操作，那么它将正常工作：

has_and_belongs_to_many :tags, autosave: true # Refer to tag_ids