How can I create unique IDs for embedded documents in MongoDB?

37
votes

So I need to reference particular subdocuments uniquely from items in my collection. For instance:

User = {
    'name': 'jim',
    'documents: [
        {'id': 0001, 'title': "My document"},
        {'id': 0002, 'title': "My second document!"},
    ]
}

So I need to be able to auto-create IDs for new documents, preferably not at the application level (since there will be race conditions in the actual development scenario).

Is there a way to use mongo's autogenerated ObjectId (used in the _id field at the collection level), or something similar?

6
mongodb
Not if you use a Guid there won't – Tony Hopkinson

6 Answers

29
votes

Yes, using mongo's ObjectId is the way to go. The only thing is: you have to generate them yourself, in the application code. They are meant to be globally unique, different workers won't generate two identical ObjectIds, so there's no race condition in that sense.

All official drivers should provide a way to generate ObjectId. Here's how it is in Ruby:

oid = BSON::ObjectId.new
11
votes

All drivers have functionality for generating ObjectIds.

In the shell you just do new ObjectId():

> db.test.insert({x:new ObjectId()});
> db.test.find();
{ "_id" : ObjectId("4f88592a06c05e4de90d0bc1"), "x" : ObjectId("4f88592a06c05e4de90d0bc0") }

In Java it's new ObjectId() as well. See the API docs for your driver to see the specific syntax.

7
votes

With mongoengine create a ObjectId in an embedded document like this:

from bson.objectid import ObjectId

class Address(EmbeddedDocument):
    _id = ObjectIdField( required=True, default=lambda: ObjectId() )
    street = StringField()
3
votes

In Meteor, on the server, use:

new Meteor.Collection.ObjectID(hexString);

See: http://docs.meteor.com/#collection_object_id

1
votes

And this is how you can do it in python (pymongo):

from pymongo import MongoClient
import bson

client = MongoClient('mongodb://localhost:27017/')
db = client.test_db

result=db.users.insert_one({'name': 'jim',
    'documents': [
        {'_id': bson.objectid.ObjectId(), 'title': "My document"},
        {'_id': bson.objectid.ObjectId(), 'title': "My second document!"},
    ]})

print list(db.users.find({}))
0
votes

And here's how to do it in Clojure, assuming the use of congomongo:

(import org.bson.types.ObjectId)
(str (ObjectId.)) ; => "12345xxxxx"