Mongodb – How to create unique IDs for embedded documents in MongoDB

mongodb

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?

Best Answer

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

Related Solutions

Sql – How to query MongoDB with “like”

That would have to be:

db.users.find({"name": /.*m.*/})

Or, similar:

db.users.find({"name": /m/})

You're looking for something that contains "m" somewhere (SQL's '%' operator is equivalent to regular expressions' '.*'), not something that has "m" anchored to the beginning of the string.

Note: MongoDB uses regular expressions which are more powerful than "LIKE" in SQL. With regular expressions you can create any pattern that you imagine.

For more information on regular expressions, refer to Regular expressions (MDN).

Mongodb – How to get the last N records in mongodb

If I understand your question, you need to sort in ascending order.

Assuming you have some id or date field called "x" you would do ...

.sort()

db.foo.find().sort({x:1});

The 1 will sort ascending (oldest to newest) and -1 will sort descending (newest to oldest.)

If you use the auto created _id field it has a date embedded in it ... so you can use that to order by ...

db.foo.find().sort({_id:1});

That will return back all your documents sorted from oldest to newest.

Natural Order

You can also use a Natural Order mentioned above ...

db.foo.find().sort({$natural:1});

Again, using 1 or -1 depending on the order you want.

Use .limit()

Lastly, it's good practice to add a limit when doing this sort of wide open query so you could do either ...

db.foo.find().sort({_id:1}).limit(50);

db.foo.find().sort({$natural:1}).limit(50);