see a code example:

There are three models , as this:

class User
  include Mongoid::Document
  embeds_many :projects
end

class Project
  embedded_in :user
  embeds_many :tasks
end

class Task
  embedded_in :project
end

Now create them to use:

user = User.new
project = Project.new
task = Task.new
project.tasks << task
user.projects << project
user.save

Then query:
{{user.reload.projects[0].tasks }}
output
[] !!

I hope it should output [task]

I found this describe about this:
Append one or many child addresses, saving them if the person is persisted

Thx.