Entity Framework: Setting a Foreign Key Property

问题:

We have a table that looks roughly like this:

CREATE TABLE Lockers 
{
  UserID int NOT NULL PRIMARY KEY (foreign key),
  LockerStyleID int (foreign key),
  NameplateID int (foreign key)
}

All of the keys relate to other tables, but because of the way the application is distributed, it's easier for us to pass along IDs as parameters. So we'd like to do this:

Locker l = new Locker { 
  UserID = userID, 
  LockerStyleID = lockerStyleID, 
  NameplateID = nameplateID 
};
entities.AddLocker(l);

We could do it in LINQ-to-SQL, but not EF?

回答1:

This missing feature seems to annoy a lot of people.

• Good news: MS will address the issue with .NET 4.0.
• Bad news: for now, or if you're stuck on 3.5 you have to do a little bit of work, but it IS possible.

You have to do it like this:

Locker locker = new Locker();
locker.UserReference.EntityKey = new System.Data.EntityKey("entities.User", "ID", userID);
locker.LockerStyleReference.EntityKey = new EntityKey("entities.LockerStyle", "ID", lockerStyleID);
locker.NameplateReference.EntityKey = new EntityKey("entities.Nameplate", "ID", nameplateID);
entities.AddLocker(locker);
entities.SaveChanges();

回答2:

What I've been doing to make things easy is adding the foreign key property myself in the partial class:

public int UserID
{
   get
   {
      if (this.User != null)
         return this.User.UserID;
   }
   set 
   {
      this.UserReference.EntityKey = new System.Data.EntityKey("entities.User", "ID", value);
   }
}

回答3:

You could make an extension method that constructs the entity based on these ID's.

回答4:

Using an EntityKey solves your problem ;)

alk.

回答5:

another method if you don't mind 'polluting' you db schema is to add a computed column, e.g. if you had a foreign key field FK_Customer you could define a new computed column FK_Customer_Computed which has the expression FK_Customer. When you generate\update your edmx model the field will appear like a regular field that you can then reference from you entity object.

Or wait for EF4 :)

回答6:

Following on from Dylan's answer, Alex James has written a blog on precisely this, explaining the problem in full and how to go about the partial class + property solution.

Faking Foreign Keys - EF3.5