public class Employee
    {
        [BsonId]
        public string Id { get; set; }      
        public int EmployeeId { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string Email { get; set; }
        public string Gender { get; set; }
        public List<Department.Department> Department { get; set; }
        public string Status { get; set; }
        public string ContactNo { get; set; }      
    }

public class Department
    {
        [BsonId]
        public string Id { get; set; }
        public int DepartmentId { get; set; }
        public string DepartmentName { get; set; }
        public string Description { get; set; }
        public string Status { get; set; }
        public DateTime CreateDateTime { get; set; }
        public DateTime ModifyDateTime { get; set; }
    }

using MongoDB.Driver;
using MongoDB.Driver.Builders;
using MongoDBMVC.Properties;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Web;
using System.Web.Mvc;
using Microsoft.Ajax.Utilities;
using MongoDB.Bson;
using MongoDB.Driver.Linq;

namespace MongoDBMVC.Employee
{
    public class EmployeeRepository : IEmployeeRepository
    {
        public MongoDatabase _MongoDB;
        public MongoCollection _MongoEmpCollection;
        public bool ServerIsDown = false;

        public EmployeeRepository()
        {
            var mongoClient = new MongoClient(Settings.Default.EmployeeCS);
            var server = mongoClient.GetServer();
            _MongoDB = server.GetDatabase("Employees");
            _MongoEmpCollection = _MongoDB.GetCollection("Employees");

            //test if server is up and running
            try
            {
                _MongoDB.Server.Ping();
                // Ping() method throws exception if not able to connect
            }
            catch (Exception ex)
            {
                ServerIsDown = true;
            }
            
        }



        private Employee[] _testEmployeeData = new Employee[]
        {
            new Employee()
            {
                Id = Guid.NewGuid().ToString(),
                EmployeeId = 50010,
                FirstName = "Surya",
                LastName = "Shankar",
                ContactNo = "99999999",
                Email = "Suryashankar.something.com",
                Gender = "Male",
                Department = new List<Department.Department>()
                {
                    new Department.Department()
                    {
                        Id = Guid.NewGuid().ToString(),
                        DepartmentId = 01,
                        DepartmentName = "CEO",
                        Description = "Head of the Office",
                        Status = "Active",
                        CreateDateTime = DateTime.UtcNow,
                        ModifyDateTime = DateTime.UtcNow
                    },

                    new Department.Department()
                    {
                        Id = Guid.NewGuid().ToString(),
                        DepartmentId = 03,
                        DepartmentName = "SAP Analyst",
                        Description = "SAP Head of the office",
                        Status = "In-Active",
                        CreateDateTime = DateTime.UtcNow,
                        ModifyDateTime = DateTime.UtcNow
                    },
                    new Department.Department()
                    {
                        Id = Guid.NewGuid().ToString(),
                        DepartmentId = 02,
                        DepartmentName = "Sr.Director",
                        Description = "CEO of the Company",
                        Status = "Active",
                        CreateDateTime = DateTime.UtcNow,
                        ModifyDateTime = DateTime.UtcNow
                    },
                },

                Status = "Active"
            },
            new Employee()
            {
                Id = Guid.NewGuid().ToString(),
                EmployeeId = 50011,
                FirstName = "Vishnu",
                LastName = "Vallapu",
                ContactNo = "99999999",
                Email = "Vishnuvallapu.something.com",
                Gender = "Male",
                Department = new List<Department.Department>()
                {
                    new Department.Department()
                    {
                        Id = Guid.NewGuid().ToString(),
                        DepartmentId = 01,
                        DepartmentName = "CEO",
                        Description = "Head of the Office",
                        Status = "Active",
                        CreateDateTime = DateTime.UtcNow,
                        ModifyDateTime = DateTime.UtcNow
                    },

                    new Department.Department()
                    {
                        Id = Guid.NewGuid().ToString(),
                        DepartmentId = 04,
                        DepartmentName = "Monitoring",
                        Description = "Checks all the work done by the employees",
                        Status = "In-Active",
                        CreateDateTime = DateTime.UtcNow,
                        ModifyDateTime = DateTime.UtcNow
                    }
                },
                Status = "Active"
            },
           
        };

        private List<Employee> _employeesList = new List<Employee>();

        public IEnumerable<Employee> GetAllEmployees()
        {
            if (ServerIsDown) return null;

            if (Convert.ToInt32(_MongoEmpCollection.Count()) > 0)
            {
                _employeesList.Clear();
                var employees = _MongoEmpCollection.FindAs(typeof(Employee), Query.NE("FirstName", "null"));
                if (employees.Count() > 0)
                {
                    foreach (Employee employee in employees)
                    {
                        _employeesList.Add(employee);
                    }
                }
            }
            else
            {
                #region add test data if DB is empty

                _MongoEmpCollection.RemoveAll();
                foreach (var employee in _testEmployeeData)
                {
                    _employeesList.Add(employee);

                    Add(employee); // add data to mongo db also
                }

                #endregion
            }

            var result = _employeesList.AsQueryable();
            return result;

        }

        public Employee Add(Employee employee)
        {
            if (string.IsNullOrEmpty(employee.Id))
            {
                employee.Id = Guid.NewGuid().ToString();
            }
            _MongoEmpCollection.Save(employee);
            return employee;
        }

        public bool Delete(int ObjectId)
        {
            _MongoEmpCollection.Remove(Query.EQ("EmployeeId", ObjectId));
            return true;
        }

        public Employee GetEmployeeById(int id = 0)
        {
            if (id == 0)
            {
                throw new ArgumentNullException("id", "Employee Id is empty!");
            }
            var employee = (Employee) _MongoEmpCollection.FindOneAs(typeof(Employee), Query.EQ("EmployeeId", id));
            return employee;
        }

        public bool Update(int objectId, Employee employee)
        {
            UpdateBuilder updateBuilder = MongoDB.Driver.Builders.Update
                .Set("EmployeeId", employee.EmployeeId)
                .Set("FirstName", employee.FirstName)
                .Set("LastName", employee.LastName)
                .Set("Email", employee.Email)
                .Set("ContactNo", employee.ContactNo)
                .Set("Gender", employee.Gender)
                .Set("Department..DepartmentName", employee.Department.DepartmentName)
                .Set("Department.Description", employee.Department.Description)
                .Set("Department.Status", employee.Department.Status)
                .Set("Department.CreateDateTime", employee.Department.CreateDateTime)
                .Set("Department.ModifyDateTime", employee.Department.ModifyDateTime)
                .Set("Status", employee.Status);
            // _MongoEmpCollection.Update()

            return true;
        }

    }
}