How to tell if a record exists in Mongo collection (C#)

Given a collection of items { url: 'http://blah' }. How can I tell if a record exists where the url is "http://stackoverflow.com"?

P.s. I am communicating with the c# driver

Solution

For any of the previous suggestions to be efficient you should be sure that there is an index on the url element. Otherwise a full collection scan will be required.

If you only expect the answer to be 0 or 1, count is probably the most efficient approach. If you think the count will be very large and all you really care about is whether there is one or more, FindOne is the most efficient approach.

It probably doesn't matter that FindOne returns the whole document unless the document is actually rather large. In that case you could tell the server to only return one field (the _id seems the most likely candidate):

var query = Query.EQ("url", "http://stackoverflow.com");
var fields = Fields.Include("_id");
var res = collection.Find(query).SetFields(fields).SetLimit(1).FirstOrDefault();
if (res == null) {
    // no match found
}