如何在EF4中将新的子实体添加到父实体？

Question

I am working with VS2010, .NET 4.0, Entity Framework 4, and POCO entities. I am trying to add and remove child entities from a parent. I have a many-to-many relationship between [User] and [Companies]. So I have the following entity:

// Objects created by POCO template generator
public class User
{
    public int Id { get; set; }
    public string Username { get; set; }
    public ICollection<Company> Companies { get; set; }
}
public class Company
{
    public int Id { get; set; }
    public string Name { get; set; }
    public ICollection<User> Users { get; set; }
}

At this point in the code, the User exists but has ZERO companies. What I am trying to do is add a new child Company to the User. This code DOES save the Company but it is also creating a duplicate User parent.

public UserRepository
{
    internal ObjectContextEntities _dbContext = new ObjectContextEntities();
    public User Save(User pocoObject)
    {
        // Load the existing User which has no companies
        var loadedEntity = _dbContext.Users.Where(m => m.Id == pocoObject.Id).FirstOrDefault();

        // Add the new company from a POCO object
        loadedEntity.Companies.Add(pocoObject.Companies.Last());

        _dbContext.SaveChanges();
    }
}

How should I be adding child entities? (Without creating a duplicate relationship?) ... Also is there a simple way to update those children when performing "ApplyCurrentValues()" on the parent User?

Answer 1

This code does work and does not create duplicate users.

public class User
{
    public int Id { get; set; }
    public string Username { get; set; }
    public ICollection<Company> Companies { get; set; }

    public User()
    {
        Companies = new List<Company>();
    }
}

public class Company
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class POCOEntitiesContext : ObjectContext
{
    private ObjectSet<User> users;
    public ObjectSet<User> Users
    {
        get { return users; }
    }

    private ObjectSet<Company> companies;
    public ObjectSet<Company> Companies
    {
        get { return companies; }
    }

    public POCOEntitiesContext() : base("name=POCOEntities", "POCOEntitiesContainer")
    {
        users = CreateObjectSet<User>();
        companies = CreateObjectSet<Company>();
    }
}

using (var ctx = new POCOEntitiesContext())
{
    var loadedUser = ctx.Users.Where(u => u.Username == "Peri").First();
    loadedUser.Companies.Add(new Company() { Name = "New Company" });
    ctx.SaveChanges();
}

Answer 2

I tried a more simplified approach and I figured it out:

    var newCompany = pocoObject.Companies.Last();
    newCompany.Users = null;
    loadedUser.Companies.Add(newCompany);

It turns out because there was a reference to the parent INSIDE the child, it was propagating up and causing a new parent to be created. So from a POCO standpoint, I had to remove the parent and keep it a very simple object. (Or I could have just removed the Child's Parent reference in the edmx). But even this makes my life easier:

    loadedUser.Companies.Add(new Company() { Name = "New Company" });