MongoDB ensureIndex across collections?

Question

Is possible to call ensureIndex across collections (even for collections which have not been created yet)?

For example, let's say I have 3 collections within the same database: "london", "newyork" and "tokyo". I have a field called "employee_username" which must be unique across all present and future collections. That is, if someone called "susan" is registered as the "employee_username" within "london", the same cannot exist within the "newyork" collection. Further, if in future, I need to create a new collection called "hongkong", the "susan" cannot be used within the "employee_username" within it.

Is this possible?

Thanks!

Yes, thank you for your reply.

However, the MongoDB says "Generally, having a large number of collections has no significant performance penalty, and results in very good performance." in [http://www.mongodb.org/display/DOCS/Using+a+Large+Number+of+Collections].

The above example given is just example. I am planning to build a social database with millions of records. So - according to the documentation - I should separate into many collections as possible, for performance reasons. Therefore, if I cannot create unique index which spans across collections, then I think I will create an "Email Addresses" collection which contains only and nothing but email addresses, uniquely indexed.

Does anyone have any comments? (My approach considers performance and optimization is #1 factor.)

Thanks!

Chris Fulstow · Accepted Answer · 2011-09-14 00:45:57Z

3

MongoDB doesn't support unique indexes that span several collections, so a good approach here would be to use a single collection with a unique index on employee_username, with documents having this sort of structure:

users

{
    _id: ObjectId("47cc67093475061e3d95369d"),
    location: "newyork",
    employee_username: "susan"
},
{
    _id: ObjectId("4dc9acea045bbf04348f9691"),
    location: "london",
    employee_username: "john"
}

To create the index:

db.users.ensureIndex( { employee_username: 1 }, { unique: true });

answered Sep 14, 2011 at 0:45

Chris Fulstow

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