class School
  include Mongoid::Document
  
  embeds_many :students
  
  field :name, type: :string
  
  validates :name, presence: true
end

class Student
  include Mongoid::Document
  
  embedded_in :school
end

[5] pry(main)> school = School.new
=> #<School _id: 534eee6e6b796c75a9000000, name: nil>
[6] pry(main)> student = school.students.new
=> #<Student _id: 534eee6e6b796c75a9010000, >
[7] pry(main)> student.save
=> true
[8] pry(main)> school.persisted?
=> false
[9] pry(main)> student.persisted?
=> true
[10] pry(main)> School.count
=> 0