Entity Framework 4.3 CF many - to - many relationship saving object? - entity-framework

Question

When creating many to many relationship using EF 4.3 code first approach, I cannot save data to connecting table, also cannot any examples on how to fill this table using saving object to Icollection... Here is my example:
MODELS
public class Hospital
{
//PK
[Key]
public int Id { get; set; }
public string Name { get; set; }
public string Address { get; set; }
public string City { get; set; }
public string County { get; set; }
public string UserName { get; set; }
public string Password { get; set; }
public Guid User_Id { get; set; }
//FK
public virtual ICollection<Operator> Operators { get; set; }
}
public class Operator
{
//PK
[Key]
public int Id { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public DateTime Dob { get; set; }
public string Email { get; set; }
//FK
public virtual ICollection<Hospital> Hospitals { get; set; }
}
public class Project: DbContext
{
public DbSet<Hospital> Hospitals { get; set; }
public DbSet<Operator> Operators { get; set; }
}
CONTROLLER
public void AddOperater()
{
Hospital h = new Hospital();
h = db.Hospitals.Single(a=>a.Id ==1);
var o = new Operator();
o.FirstName = "John";
o.LastName = "Doe";
o.Dob = new DateTime(1988,2,12);
o.Email = "johndoe#gmail.com";
o.Hospitals.Add(h);
db.SaveChanges();
}
With this approach I keep getting error here: o.Hospitals.Add(h); even when my Hospital instance is filled with data. How exactly to save data to both tables, the dbo.Operators and dbo.OperatorHospital which is relationship table?

Answer 1

o.Hospitals.Add(h) will fail because the list is a null list. You cannot call Add() on a null list. Typically most people get around this by instantiating the list in the constructor of the entity... like so... the current line is blowing up due to a CSharp issue.
public class Hospital
{
//PK
[Key]
public int Id { get; set; }
public string Name { get; set; }
public string Address { get; set; }
public string City { get; set; }
public string County { get; set; }
public string UserName { get; set; }
public string Password { get; set; }
public Guid User_Id { get; set; }
//FK
public virtual ICollection<Operator> Operators { get; set; }
public Hospital()
{
Operators = new List<Operator>();
}
}
public class Operator
{
//PK
[Key]
public int Id { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public DateTime Dob { get; set; }
public string Email { get; set; }
//FK
public virtual ICollection<Hospital> Hospitals { get; set; }
public Operator()
{
Hospitals = new List<Hospital>();
}
}