Changing the primary key on a MongoDB collection - mongodb

Question

I took a shortcut earlier and made the primary key of my Mongo database by concatenating various fields to create a "unique id"
I would now like to change it to actually use the ObjectId. What's the best approach to do it? I have a little over 3M documents and would like this to be as least disruptive as possible.
A simple approach would be to bring down the site for a bit and then copy every document from one to the other one which is using ObjectIds but I'd like to keep the application running if I can. I imagine one way would be to write to both for a period of time while the migration happens but that would require me having two similar code bases so I wonder if there's a way to avoid all that.
To provide some additional information:
It's just one collection that's not referenced by any others. I have another MySQL database that contains some values that are used to create the queries that read from this MongoDB collection.
I'm using PyMongo/Mongoengine libraries to interact with MongoDB from Python and I don't know if it's possible to just change the primary key for a collection.

Answer 1

You shouldn't bring your site down for any reason if it does not go down itself. :)
No matter how many millions of records you have, the solution to the problem resides on how you use your ids.
If you cross-reference documents in different collections using these ids, then for every updated object, you will update all other objects that references this one.
As first step, your system should be updated to stop creating new objects in the old way. If your system lets you easily do this, then you can update your database very easily. If this change is not easy to make, then your system has some architectural problems and you should first change this. If this is the situation, please update your question so I can update my answer.
Since I don't know anything about your applications and data, what I say will be too general. Let's call the collection you want to update coll_bad_id. Every item in this collection is referenced in other collections like coll_poor_guy and coll_wisdom_searcher. How I would do this is to run over coll_bad_id one item at a time like this:
1. read one item
2. update _id with new style of _id
3. insert item back to collection
-- now we have two copies of the same item one with old-style id, one with new
4. update each item referencing this to use new style id
5. remove the duplicate item with old-style id from collection
One thing you should keep in mind that, bson ObjectId's keep date/time data that can be very useful. Since you rebuild all these objects on one day, your ObjectId's will not reflect correct creation times for these items. For newly added items, they will. You can note the first newly added item as the milestone of items with ids with correct-creation times.
UPDATE: Code sample to run on Mongo shell.
This is not the most efficient way to do this; but it is safe to run since we do not remove anything before adding them back with a new _id. Better can be doing this in small amounts by adding queries to find() call.
var cursor = db.testcoll.find()
cursor.forEach(function(item) {
var oldid= item._id; // we save old _id to use for removal below.
delete item._id; // When we add an item without _id, Mongo creates a unique _id.
db.testcoll.insert(item); // We add item without _id.
db.testcoll.remove(oldid); // We delete the item with bad _id.
});