request

curl http://localhost:3000/users -d '{"user": { "first_name": "Abhishek", "login_info_attributes": { "password": "p"}}}'

user.rb

class User
  include Mongoid::Document
  
  store_in collection: "dummy"

  field :first_name,  type: String
  field :last_name,   type: String
  
  embeds_one :login_info
  
  accepts_nested_attributes_for :login_info
end 

class LoginInfo
  include Mongoid::Document
  
  field :username, type: String
  field :password, type: String
  
  embedded_in :user
end

users_controller.rb

class UsersController < ApplicationController
  respond_to :json
  
  def index
    @users = User.all
    respond_with(@users)
  end
  
  def create
    @user = User.create(user_params)
    respond_with(@user)
  end
  
  private
    def user_params
      params.require(:user).permit(
        :first_name, 
        :last_name, 
        
        login_info_attributes: [
          :username, 
          :password
        ]
      )
    end
end