实体框架：避免循环引用的多次保存

Question

I'm getting this exception when calling SaveChanges on my EF ObjectContext:

Unable to determine a valid ordering for dependent operations. Dependencies may exist due to foreign key constraints

I think the problem is because I have a circular dependency on my DB model.

ie

Table Users

• Id
• ProfilePictureId

Table Pictures

• Id
• UserId

I'm creating a new user and setting the picture

var user = _db.Users.CreateObject();
var picture = _db.Pictures.CreateObject();

picture.User = user;
user.ProfilePicture = picture;

_db.SaveChanges();

But that throws the exception.

If I add an extra call to SaveChanges() after I set the picture's User It works just fine, I just want to avoid that double trip to the DB.

Any ideas of how to achieve this?

Thanks!

Answer 1

There is no way to avoid calling SaveChanges twice with your database design. You can't insert user with dependency to picture which is not inserted yet (FK will throw exception) and in the same time you can't insert picture with dependency to user which is not inserted yet (again FK will throw exception). That is not feature of EF that is feature of DB itself.

You also don't need to avoid multiple SaveChanges calls because of multiple roundtrips. EF doesn't have command batching so each insert, update or delete has its own roundtrip to database anyway.

If you want to call single SaveChanges you must change your database as follows:

Table Users

• Id (PK, IDENTITY)

Table Pictures

• Id (PK, FK to Users.Id, No IDENTITY)

This is native one-to-one relation where User is principal and Picture is dependent.

Answer 2

I would say that it's enough to make the assignment once (either setting user or picture) ie:

var user = _db.Users.CreateObject();
var picture = _db.Pictures.CreateObject();

user.ProfilePicture = picture;

_db.SaveChanges();

You are creating new entities at first two lines, and you 'link' them by user.ProfilePicture = picture . EF should handle the rest.
You don't need to setup relation from both sides.

Edit: What about this?

var user = _db.Users.CreateObject();
var picture = _db.Pictures.CreateObject();

picture.user = user;
user.ProfilePictureId = picture.Id;


_db.SaveChanges();