One to many relationship with objects using EF core

Question

I have four entities:

Employee: It belongs to Group and Department entities Group: Contains a list of Employees Department: Contains a list of Employees Todo: Assigned to a Group

The thing is: I tried to add FK to the tables and like 10 other approaches but I just can't figure it out. Let's see for example Department:

public class Department
    {
        public Department()
        {
            Employees = new List<Employee>();
        }

        [Key]
        public int DepartmentId { get; set; }

        [Required]
        public string Name { get; set; }

        [Required]
        public string Field { get; set; }

        public ICollection<Employee> Employees { get; set; }
    }

And here is the Employee entity:

public class Employee
    {
        public int EmployeeId { get; set; }

        [Required]
        [MaxLength(50, ErrorMessage = "Name cannot exceed 50 characters")]
        public string Name { get; set; }

        [Required]
        [RegularExpression(@"^[a-zA-Z0-9_0+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$", ErrorMessage = "Invalid Email Format")]
        [Display(Name = "Office Email")]
        public string Email { get; set; }

        [ForeignKey("Department")]
        public int DepartmentId { get; set; }

        public Department Department { get; set; }

        [ForeignKey("Group")]
        public int GroupId { get; set; }

        public Group Group { get; set; }
    }

I generated a simple controller and view page for Departments and in the details method I try to print out the department.Employees.Count() but it says it's 0.

Here is my appDbContext:

modelBuilder.Entity<Employee>()
                .HasData(
                new Employee() { EmployeeId = -99, Name = "Mary", Email = "mary@gmail.com", DepartmentId = -99, GroupId = -1 },
                new Employee() { EmployeeId = -98, Name = "Stan", Email = "stan@gmail.com", DepartmentId = -99, GroupId = -1 },
                new Employee() { EmployeeId = -97, Name = "Mike", Email = "mike@gmail.com", DepartmentId = -99, GroupId = -1 });

            modelBuilder.Entity<Department>()
                .HasData(
                new Department() { DepartmentId = -99, Field = "IT", Name = "Programming Department" },
                new Department() { DepartmentId = -98, Field = "HR", Name = "Human Resorcues" },
                new Department() { DepartmentId = -97, Field = "AD", Name = "Advertisement Department" });

Answer 1

Your question has a lot of "holes" and you must still trying and learning. So this question will be updated if you need more asistance.

I think you are having troubles to understand objects.

If you want to do it in your eg, the departament has a list of employees (as I can see in your statement):

But:

   public List<Employee> Employees 
    {
      get; set;
    }

and then:

 new Department() { DepartmentId = -99, Field = "IT", Name = "Programming Department", Employees = x },

where x is an instance or List that keep or is this itself:

modelBuilder.Entity<Employee>()
                .HasData(
                new Employee() { EmployeeId = -99, Name = "Mary", Email = "mary@gmail.com", DepartmentId = -99, GroupId = -1 },
                new Employee() { EmployeeId = -98, Name = "Stan", Email = "stan@gmail.com", DepartmentId = -99, GroupId = -1 },
                new Employee() { EmployeeId = -97, Name = "Mike", Email = "mike@gmail.com", DepartmentId = -99, GroupId = -1 })

But for database is more easy with EF:

//Save your employee in the context.
_context.Employees.Add(newemployee);
//Set the employee in the departament
thedepartament.Employees.Add(newemployee); 
//Save everything
_context.SaveChanges();

UPDATE:

Of course you won't create a departament everytime you "add" a new "employee", the above example is only for insert a relation child at the same time. So what you need is thinking in this way:

//Save your employee in the context.
_context.Employees.Add(newemployee);
//Search for the departament you want to add this employee, with a query:
Departament thedepOfTheNewEmployee = _context.Departament.Where(your conditions to search it).FirstOfDefault();
//Set the employee to that departament
newemployee.DepartmentId = thedepOfTheNewEmployee.Id;
//Save everything
_context.SaveChanges();

More further theory:
If you Add your element to the context, will be provided (virtually) an Id to retrieve. This theory is not for your scenario, but will help you in the future.