Entity Framework update virtual property directly without creating new record

Here is a simple entity:

public class Customer : Entity
{
    public virtual Location Location { get; set; }
}

Now suppose we have a customer already:

var customer = new Customer() {Location = new Location("China")};

and now we want to update his location:

var customer = context.Customers.First(x => x.Location.Country == "China");
customer.Location = new Location("America");
context.SaveChanges();

and now when I look at the database, the location record "China" has not been deleted: the database now has two location records association with one customer record.

The reason for this issue is that I'm using virtual keyword on the Customer.Location property, and when I query the customer entity from database I didn't use Include method to load the Location property, and I also did not use any accesses to lazy load it. So EF can't track and know the China Location entity should be deleted.

I think the approach I used to the update virtual property is in line with intuition. I want update a property, then just use a update instruction "entity.xxx=...", add being forced to use some access of the property or method call while loading "entity.xxx" is not intuitive.

So I am looking for some better way to replace an entity's virtual property directly. Any suggestions?

---

Solutions update

I'm find two way to do this easy,

First you can use Identifying relation( recommend ).

Another way you can Use ObjectContext.DeleteObject method, below is the example code:

public static class EFCollectionHelper
{
    public static void UpdateCollection<T, TValue>(this T target, 
Expression<Func<T, IEnumerable<TValue>>> memberLamda, TValue value)where T : Entity
    {
        var memberSelectorExpression = (MemberExpression)memberLamda.Body;
        var property = (PropertyInfo)memberSelectorExpression.Member;

        var oldCollection = memberLamda.Compile()(target);
        oldCollection.ClearUp();

        property.SetValue(target, value, null);
    }

    public static void ClearUp<T>(this IEnumerable<T> collection)
    {
        //Convert your DbContext to IObjectContextAdapter
        var objContext = ((IObjectContextAdapter) Program.DbContext).ObjectContext;
        for (int i = 0; i < collection.Count(); i++)
        {
            objContext.DeleteObject(collection.ElementAt(i));
        }
    }
}

And then you can simply write the code like:

customer.UpdateCollection(x => x.Locations, null);

---

Not completely sure what you want, but this is what i got.

The reason for you now getting two locations are because you use new Location("American"); you actually add a reference to a new location (EF don't know if China is used by another customer, and would never delete it, in that type of query)

Now if you said.

customer.Location.Country = "America"

The China would be overwritten by America , as we are now working with a specific Location 's property.

Read the coments on the question so a little extras

If you want to update the location fully ( new Location("Some new location") ). Then you would do it like this.

Location oldLocation = customer.Location;
Location newLocation = new Location("America");
//Check if the new location country !exist
if(!context.Locations.Any(a=> a.Country == newLocation.Country))
{
    //If it don't exist then add it (avoiding the location duplicate)
    customer.Location = newLocation;
    //If its important to delete the old location then do this
    //(important to do after you removed the dependency, 
    //thats why its after the new location is added)
    context.Locations.Remove(oldLocation)
    //Finally Save the changes
    context.SaveChanges();
}

---

另一种更新实体的方法是使用Entry.OriginalValues.SetValues方法：

var currentLocation = context.Customers.First(x => x.Location.Country == "China").Select(c => c.Location);
context.Entry(currentLocation).OriginalValues.SetValues(newLocation) ;
context.SaveChanges();

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