Do a query for uniqueness in MongoDB and insert if unique

Question

I'm totally new to MongoDB. I'm using Morphia to access it from Java. I need to do something along the lines of this:

public boolean isUnique(short s){
//OPERATION 1: Check in mongo if object with field "id" = s exists
//if so, return false
//else,
//OPERATION 2: create this object in the database
//and return true
}

The problem for me to grasp is not the actual syntax, but the problem with atomicity. How do I assure that only one thread/process can have access to the document so that OP1 and OP2 are atomic? I think this has to be managed on a database level since the Java server is in a clustered environment.

Regards, Anders

Answer 1

You could replace both operations with one upsert of the {id: s} document that becomes a no-op if the document already exists, but an insert if it doesn't. Calling the getLastError command (the driver can do this for you) will tell you whether a new document was created or not.