mongoid atomic updates in rails

i am having an issue with updating existing documents composite key fields in rails. the associated mongo query for my u开发者_如何学Cpdate_attributes statement seems to be correct, however the object cannot be found afterwards.

for example with an existing object with first_name "Jane" and last_name "Doe"... with my :key being :first_name, :last_name

i hit my update method with:

{"artist"=>{"last_name"=>"Doe", "first_name"=>"John"}, "commit"=>"Update Artist", "id"=>"jane-doe"}

  def update
    @artist = Artist.find(params[:id])
    if @artist.update_attributes(params[:artist])
      redirect_to @artist
    end
  end

which generates the mongo query: MONGODB app_database['artists'].update({"_id"=>"jane-doe"}, {"$set"=>{"_id"=>"john-doe", "first_name"=>"John"}})

which seems correct to me... but when i am redirected to that new artist, it complains about Document not found for class Artist with id(s) john-doe.

and in fact looking at my db from the mongo console, i still see Jane Doe in there. It is something to do with them being composite key fields, since i can update non-key fields just fine.

what am i missing here?

I tried this in an app of my own, and it looks like MongoDB simply doesn't currently allow you to modify the _id field as part of a $set operation (which is what Mongoid uses to perform updates). This is a strange restriction - I've been using Mongo for a year and a half now and I've never run into it.

So, a few options:

• Stop using anything for your _id that might need to be changed later. I'd do this - it's a best practice anyway.

• Whenever you need to make one of these _id changes, instead create a new record with the new _id attribute and delete the old one. This might get messy though, especially if you have other models that refer your Artist models by id.

• File an issue with 10gen asking for this restriction to be lifted. They're very good about responding to users' concerns, but even if they agree, it'll probably take a while to be done.

• File an issue with Mongoid to request support for these types of changes (Mongoid could conceivably handle the create + delete mechanism for you), but honestly, it's the kind of edge case that's probably not worth the extra time and code for them to support. It'd be nice if Mongoid raised an error when you tried to do an update like this, at the very least.