Question

I am using EF5 codefirst in an ASP.NET MVC4 app. I have products and partners, and a number of partners can be assigned to a product.

In the Product entity I have this:
public ICollection Partners { get; set; }

And in the Partner entity I have this:
public ICollection Product { get; set; }

And thus in my sql server a PartnerProducts many-2-many table is created by code first.

I then have this action method:

    public ActionResult AssignPartner(long productId,  long partnerId) {
        var product = productRepository.Find(productId);
        var partner = partnerRepository.Find(partnerId);
        if (product.Partners == null) {
            product.Partners = new List<Partner>();
        }
        product.Partners.Add(partner);
        productRepository.Save();
        return RedirectToAction("Edit", new{ Id = productId });
    }

But the result is that both a new PartnerProducts row is created (ok) and also a new partner is created in the Partners table ? Somehow EF must think that the partner I add is a new record?

What am I missing here?

Answer 1

Try this:

public ActionResult AssignPartner(long productId,  long partnerId)
{
    var product = productRepository.Find(productId);
    var partner = partnerRepository.Find(partnerId);

    // Using Attach - partner will be in state Unchanged in the context
    dbContext.Partners.Attach(partner);

    if (product.Partners == null)
    {
        product.Partners = new List<Partner>();
    }

    product.Partners.Add(partner);

    productRepository.Save();

    return RedirectToAction("Edit", new{ Id = productId });
}