Hi,

I've got two models with a HABTM relationship:

class Article
include Mongoid::Document
include Mongoid::Timestamps

has_and_belongs_to_many :tags

field :title, :type => String
end

and

class Tag
include Mongoid::Document
include Mongoid::Timestamps

has_and_belongs_to_many :articles
end

If I create a new article using #create everything works fine:

Article.create(:title => "Test", :tag_id => Tag.first.id)

But if I'm using #save, the tag model doesn't know anything about it related articles

article = Article.new(:title => "Test", :tag_id => Tag.first.id)
article.save

I'm using Rails 3.1 and Mongoid 2.2.0. This bug does not exist in Mongoid 2.0.2.