How can I change the type of objects' _id field to ObjectId in the MongoDB shell?

Question

I have a few objects in a MongoDB database where the _id field was assigned an integer by mistake, how can I in the MongoDB shell replace these integers with an ObjectId?

An example of such an object would look like this:

{ "_id" : 0 }

Answer 1

The best solution I found was to clone each each object to another with an ObjectId value assigned to _id, and then delete all objects with an integer for _id. I did the following in the shell:

// $type: 16 means int32
> db.Roles.find({_id: {$type: 16}}).forEach(function (x) {
          x._id = ObjectId();
          db.Roles.save(x);
      })
> db.Roles.remove({_id: {$type: 16}})