MongoDB: find document with composite _id

Question

I have documents in my collection with a composite _id like this:

_id:{a:"", b:""}

What that I want is to find the document knowing only the value of the field a. Is there some way to do this?

Thanks!

Answer 1

Sure you can do

db.whatever.find({"_id.a":"valueOfa"});

Or did I not got the question?