class Post
    include Mongoid::Document
    embeds_many :comments
end

class Comment
    include Mongoid::Document
    field :text, :type => String
    embedded_in :post
end

Post.delete_all
Post.create!(:comments => [{ :text => 'message 1' }])
p = Post.first
p.update_attributes :comments => [{ :text => 'message 2' }]

Post.first.comments.count
=> 1

class Blog
    include Mongoid::Document
    embeds_many :posts
end
class Post
    include Mongoid::Document
    embedded_in :blog
    embeds_many :comments
end

class Comment
    include Mongoid::Document
    field :text, :type => String
    embedded_in :post
end

Blog.delete_all
Blog.create!({ :posts => [{ :comments => [{ :text => 'message 1' }] }] })
p = Blog.first.posts.first
p.update_attributes :comments => [{ :text => 'message 2' }]
reload!
Blog.first.posts.first.comments.count
=> 2