gistfile1.js

> db.groups.findOne()
{
        "_id" : 1,
        "users" : [
                {
                        "name" : "skot",
                        "age" : 1
                },
                {
                        "name" : "fred",
                        "age" : 2
                }
        ]
}
> db.groups.findOne({users:{$exists:true}})
{
        "_id" : 1,
        "users" : [
                {
                        "name" : "skot",
                        "age" : 1
                },
                {
                        "name" : "fred",
                        "age" : 2
                }
        ]
}
> db.groups.findOne({users:{$exists:false}})
{ "_id" : 2, "v" : [ 1, 2, 3 ] }