Entity-Framework Many-Many - entity-framework

Question

So I have Work and Person models as follows:
public class Work{
public int ID
public string Name
public virtual ICollection<Person> CurrentWorkers
public virtual ICollection<Person> ProjectOwners
}
public class Person{
public int ID
public string UserID
public virtual ICollection<Work> CurrentWork
}
Now when I go into the db to get my person (whom submitted the Work request):
//if exists
var person = db.Person.Find(id)
//else
Person person = new Person()
and try to assign an owner to the Work:
Work work = new Work()
work.ProjectOwners.Add(person) //NULL REF ERROR HERE
db.Work.Add(work)
I get a Null Reference error, which I guess is because work isn't in the db yet? So do I have to insert work and then go find it and then attach the Person? I don't quite understand how EF Many-Many works and links for additional reading into this would also be appreciated.
UPDATE
So the below works once but when I create a new Work, whatever was saved with any previous work.ProjectOwners gets wiped out. Any help with this, I understand WHY (because the FK column in People is being overwritten with the next Work PK) but I have no idea how to fix it?
work.ProjectOwners = new List<Person>();

Answer 1

Take a look at this example:
public class Work
{
public int WorkId { get; set; }
public string Name { get; set; }
public virtual ICollection<Person> CurrentWorkers { get; set; }
public virtual ICollection<Person> ProjectOwners { get; set; }
}
public class Person
{
public int PersonId { get; set; }
public string UserId { get; set; }
public virtual ICollection<Work> CurrentWork { get; set; }
}
public class WorkContext : DbContext
{
public DbSet<Work> Works { get; set; }
public DbSet<Person> Persons { get; set; }
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<Work>().
HasMany(w => w.ProjectOwners).
WithMany(p => p.CurrentWork).
Map(
m =>
{
m.MapLeftKey("WorkId");
m.MapRightKey("PersonId");
m.ToTable("WorkPersons");
});
}
}
And here a simple test:
var personJohn = new Person() {UserId = "John Doe"};
var personJane = new Person() { UserId = "Jane Dee" };
var work = new Work();
work.ProjectOwners = new List<Person>();
work.ProjectOwners.Add(personJohn);
db.Works.Add(work);
db.SaveChanges();
work.ProjectOwners.Add(personJane);
db.SaveChanges();
Edit:
Variation where you add a Person that already exists in the database (e.g. Jane):
var john = new Person() {UserId = "JohnDoe"};
var jane = new Person() { UserId = "JaneDee" };
//Add Jane to DB
db.Persons.Add(jane);
db.SaveChanges();
//Create new Work and initialize ProjectOwners collection
var work = new Work();
work.ProjectOwners = new List<Person>();
//Add John (doesn't exist in DB yet)
work.ProjectOwners.Add(john);
db.Works.Add(work);
db.SaveChanges();
//Look for Jane in DB
jane = db.Persons.FirstOrDefault(x => x.UserId == "JaneDee");
if (jane != null)
{
//Add existing Jane if found to work Project Owners
work.ProjectOwners.Add(jane);
}
db.SaveChanges();