Many to one on same mongoid collection

seed.rb

admin = User.create! :first_name => 'admin', :last_name => 'admin', :email => 'admin@example.com', :mobile => '9999911111', :locality => 'Goregaon', :city => 'Mumbai', :user_type => 'admin', :username => 'admin@example.com', :password => 'admin' 

franchisee = User.create! :first_name => 'franchisee', :last_name => 'franchisee', :email => 'franchisee@example.com', :mobile => '9999911111', :locality => 'Goregaon', :city => 'Mumbai', :user_type => 'franchisee', :username => 'franchisee@example.com', :password => 'franchisee'

franchisee.parent = admin
franchisee.save!

user.rb

class User
  include Mongoid::Document
  store_in collection: "users"
  validates :first_name,:last_name,:email,:mobile,    presence: true
  validates :locality,:city,                          presence: true
  
  belongs_to :parent,   class_name: "User"
  has_many   :children, class_name: "User"

  field :first_name,    type: String
  field :last_name,     type: String
  field :email,         type: String
  field :mobile,        type: String
  field :locality,      type: String
  field :city,          type: String
  field :address,       type: String
  field :blood_grp,     type: String
  field :emergency_no,  type: String
  field :user_type,     type: String
  field :dob,           type: Integer  
  field :username,      type: String
  field :password,      type: String
end