MongoDB c# driver _id field and SetIdMember()

Question

I have set the SetIdMember() for a class map to point to an element within the object:

BsonClassMap.RegisterClassMap<Person>(x =>
    {
        x.AutoMap();
        x.SetIdMember(x.GetMemberMap(p => p.PersistedId));
    });

Which works fine, however I was looking in the mongo data (via MongoVUE) and noticed that the PersistedId field is not stored anywhere in the database, I presume this is because it maps it to the _id field.

Now for 99% of scenarios im sure that is fine, however in this case I am using a non-strongly typed model, and pulling back a raw BsonDocument, then turning it to JSON for handing to a clientside script to then use further. However it is expecting to get a PersistedId field in the JSON but it doesn't exist, it is just _id.

So is there a way for me to get it to do its whole unique _id field thing, but also get it to write out the PersistedId field to the database too? (I know its going to be duplicated data, but its not a major worry)

Answer 1

You can add a readonly property and let the driver know to store it in the database as PersistedId.

public class Entity
{
    public ObjectId Id { get; set; }

    public ObjectId PersistedId 
    {
        get { return Id; }
    }
}

BsonClassMap.RegisterClassMap<Person>(x =>
{
    x.AutoMap();
    x.MapMember(x => x.PersistedId);
});

However, I'm not sure this is the best idea. You likely shouldn't be sharing your database models with your UI in a server environment like this. Rather, you should be mapping from your database models to UI models. This provides a large number of benefits, such as aggregating models, transforming models, etc... so that your UI isn's so coupled to your UI. But, ultimately, it is your call.