Question

I have a File entity and a User entity. The File entity has a 1:1 relationship with the User entity through a property called LastChangeUser (this records the user who last changed the file). There’s also a field within the File entity named LastChangeUserId, which is the actual FK relationship. The relationship is one-way: the User entity has no navigation property leading back to the File entity.

class File
{
    public int Id { get; set; }

    public int? LastChangeUserId { get; set; }
    public virtual User LastChangeUser { get; set; }
}

class User
{
    public int Id { get; set; }
}

When a File is changed, I need to set the LastChangeUser for the File. I only have the ID of the user to hand, not the complete User object. So, I’m doing this:

file.LastChangeUser = null;
file.LastChangeUserId = userId;

This seems to work on creating the file, when the File object is newly-created (a POCO which is then added to the entity collection).

However, it does not work when updating the file, when the File object is an existing object retrieved (as a proxy) from the DB.

In the latter case, I end up with a NULL in the DB for the LastChangeUserId field. (After a call to SaveChanges, the object has null in both the LastChangeUser and LastChangeUserId fields).

Maybe I’m just doing the wrong thing here? What’s the right way? Do I really need to go get the User object in order to set the LastChangeUser property?

Answer 1

The reason @kevin_fitz solution works here is due to the way changetracking and validation in EF work. The default behavior for change tracking in EF is a method called snapshot tracking which essentially clones an initial state of every entity when its first loaded. When you go to save changes in EF the original snapshot for each entity is compared to the current state object (the one which you are modifying) and any differences are persisted to the database.

Along side this EF also performs pre-submit validation on entities (which can FYI be disabled).

In your case you have made two changes to the model which will be detected by the snapshot tracker on save (and they actually conflict). The tracker however will try and process both of these through the validation rules which will pickup that this is a required relationship and cant be set to null. This is why you are seeing this error and why removing the null update fixes your problem.

On a sidenote, you actually only need to update either the object or the key on a navigation property to trigger that database relationship to be updated. For more details on how navigation properties work in EF codefirst checkout my article here: http://blog.staticvoid.co.nz/2012/07/entity-framework-navigation-property.html