Upserting in Mongo DB and the Id problem

Yurii Hohan picture Yurii Hohan · Sep 2, 2011 · Viewed 17.7k times · Source

I have a problem while upserting to mongo db using the official C# driver.

public abstract class AggregateRoot
{
    /// <summary>
    /// All mongoDb documents must have an id, we specify it here
    /// </summary>
    protected AggregateRoot()
    {
        Id = ObjectId.GenerateNewId();
    }

    [BsonId]
    public ObjectId Id { get; set; }
}

My entities already have the id-s but I had to create the mongo specific Id for it to work, as all the documents in a collection should have one. Now then I receive a new entity in my system a new Mongo Id is generated and I get the mongo cannot change _id of a document old exception. Is there some work-around?

Let me describe the design a bit. All the entities which would be stored as documents were inheriting from AggregateRoot which had the id generation in it. Every sub-document had its id generated automatically and I had no problem with this. The id in AggregateRoot was introduced to correct the problem when retrieving data from MongoCollection to List and the generation was introduced so the id-s are different. Now we can move that id generation to save methods because the new entity for update had a new id generation. But it would mean that every dev on the team must not forget generating id-s in repository which is risky. It would be nicer just to ignore the id than mapping from mongo if it is possible and not to have AggregateRoot class at all

Answer

renadeen picture renadeen · Nov 29, 2012

I've encountered similar problem. I wanted to upsert documents using official C# driver. I had a class like this:

public class MyClass
{
    public ObjectId Id { get; set; }
    public int Field1 { get; set; }
    public string Field2 { get; set; }
}

In console I would write: db.collection.update({Field1: 3},{Field1: 3, Field2: "value"}) and it would work. In C# I wrote:

collection.Update(Query.EQ("Field1", 3),
                Update.Replace(new MyClass { Field1 = 3, Field2 = "value" }),
                UpdateFlags.Upsert);

and it didn't work! Because driver includes empty id in update statement and when I upsert second document with different value of Field1 exception E11000 duplicate key error index is thrown (in this case Mongo tries to insert a document with _id that already exists in db).

When I generated _id by myself (like topic starter) I've encountered the same exception (mongo cannot change _id of a document) on upserting objects with existing value of Field1.

Solution is to mark Id property by attribute [BsonIgnoreIfDefault] (and not initialize it). In this case driver omits _id field in update statement and MongoDb generates Id if it necessary.