Entity Framework principal end of an association error

I have two models

class Employee {
 [Key]
 public int ID {get;set;}
 public string FirstName {get;set;}
 public string LastName {get;set;}
 public int EmploymentID {get;set;}

 [Required, ForeignKey("Employment")]
 public virtual Employment Employment {get;set;}

}

class Employment {
 [Key, ForeignKey("Employee")]
 public int ID {get;set;}
 public string Department {get;set;}
 public int OfficePhone {get;set;}
 public int EmployeeID {get;set;}

 public virtual Employee Employee {get;set;}

}

basically each employee has employment information in the Employment class. I'm not sure if I need the [Required] annotation there, and I don't know if I'm putting the [ForeignKey] annotation in the right spot either.

The problem is, when I try to create a new scaffolded item, it gives me this error:

Unable to determine the principal end of an association between the types 'Bla.Models.Employee' and 'Bla.Models.Employment'. The principal end of this association must be explicitly configured using either the relationship fluent API or data annotations.

Thank you for the help

EDIT

assume the Employee model has the following instead:

class Employee {
 [Key]
 public int ID {get;set;}
 public string FirstName {get;set;}

 //note the return value is an ICollection object
 public ICollection<LastName> LastName {get;set;}
 public int EmploymentID {get;set}

 public virtual Employment Employment {get;set;}
}

and the Employment model remains the same, and the LastName field has the following class

class LastName {
 public string EmployeeLastName {get;set;}
 //assuming last name can change, and employee had a different last name at some point
 public int year{get;set;}
}

is it incorrect to make the LastName class, a model? or should it remain a model? how can I make it so that its just a resource class (i.e. not a model to be made into a table) further, will this kind of thing break the relationships between employee/employment models?

because I'm still getting the error and not sure why; by the way, I have many of these classes like the "LastName" example and they are all currently under models, and I'm not sure if they should be a model or some resource class, and if so, I don't know where they are supposed to go

also heres my dbcontext

public class MyDbContext : DbContext
    {
        public MyDbContext()
            : base("MyDbContext")
        {
        }

        public DbSet<Employee> Employees { get; set; }
        public DbSet<Employment> Employments { get; set; }
        public DbSet<BasicAddress> Adresses { get; set; }
        public DbSet<Department> Departments { get; set; }
        public DbSet<BasicDate> BasicDate { get; set; }
        public DbSet<BasicPhoneNumber> BasicPhoneNumber { get; set; }
        public DbSet<EmployeeIdentification> EmployeeIdentification { get; set; }
        public DbSet<FederalIdentification> FederalIdentification { get; set; }
        public DbSet<JobTitle> JobTitle { get; set; }
        public DbSet<LastName> LastName { get; set; }
        public DbSet<MaritalStatus> MaritalStatus { get; set; }
        public DbSet<OfficeLocation> OfficeLocation { get; set; }
}

everything other than employee and employment is a basic class (like the LastName class), I'm not sure if they should be with "models" and made into tables or just be regular classes on the side. If they shouldn't be made into tables, where should they go in the project?

thank you again for the help! (please let me know if anything needs clarification)

Solution

Employment cannot exist with Employee. As per the law of foreign relationship the Employment must not be saved without Employee. Thus with the help of [ForeignKey] notation you need to advise Employment that Employee relation should be maintained.

Employment is dependent on Employee. Thus you got to tell Employment that you are got to have a linking with the principal.

Thus dependent class must have Employee id reference.

using System;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Linq;

namespace TestConsole
{
    public class Employee
{
    public int ID { get; set; }

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

    public string LastName { get; set; }

    [Required]
    public virtual Employment EmploymentDetails { get; set; }
}

public class Employment
{
    [Key, ForeignKey("Employee")]
    public int ID { get; set; }

    public string Department { get; set; }

    public int OfficePhone { get; set; }

    public virtual Employee Employee { get; set; }
    }


    internal class Program
    {
        public TestConsoleDbContext MyDbContext { get; set; }

        public Program()
        {
            MyDbContext = new TestConsoleDbContext();
        }

        private static void Main(string[] args)
        {

            var program = new Program();

            var records = from p in program.MyDbContext.Employees
                          select new { p.EmploymentId, p.LastName, p.Employment.Department };

            foreach (var r in records)
            {
                Console.WriteLine("EmploymentID: {0} {1} Department: {2}", r.EmploymentId, r.Department);
            }

            Console.ReadLine();


        }
    }
}

using System.Data.Entity;

namespace TestConsole
{
    internal class TestDbContext : DbContext
    {
        public IDbSet<Employee> Employees { get; set; }
        public IDbSet<Employment> Employments { get; set; }
    }
}