Question

is there a way to change a MongoDB collection reference?
With change I mean the value in the reference.
I can’t just create a direct link because the database is in use.
My structure looks like this

{
        "_id" : ObjectId("4e7c6b47e4b0dea06288ad21"),
        "license" : "ABC123",
        "model" : "911",
        "make" : "Porsche",
        "owner" : {
                "$ref" : "users",
                "$id" : "Test User"
        }
}

I want to change the ID like

{ "$ref" : "users", "$id" : "NEW USER"}

Answer 1

Sure you can, but you can’t do it with a direct $set :

> db.test.save({a:{$ref:"users", $id:"Test User"}})
> db.test.find()
{ "_id" : ObjectId("4f1e85489d086ee4511551b6"), "a" : DBRef("users", "Test User") }
> db.test.update({}, {$set:{'a.$id':"NEW USER"}})
> db.test.find()
{ "_id" : ObjectId("4f1e85489d086ee4511551b6"), "a" : { "$id" : "NEW USER", "$ref" : "users" } }

As you can see this breaks the references because the DBRef specification demands the order of the $ref and $id fields to be $ref first.

You can update it like this :

> db.test.update({}, {$set:{a:{$ref:"users", $id:"NEW USER"}}})
> db.test.find()
{ "_id" : ObjectId("4f1e858b9d086ee4511551b7"), "a" : DBRef("users", "NEW USER") }

Or just use “new DBRef(..)” if you’re in shell.