Update a value in MongoDB based on its current value

I want to do something like开发者_StackOverflow社区 this, but the this keyword doesn't seem to be set inside the update statement.

db.items.update({foo:…}, {$set: {bar: this.foo}}, false, true)

Do I have to use eval to accomplish this?

Yes, you can't refer to other fields in the modifier. You'll have to use db.eval.