Finding max field value in mongodb using node - node.js

Question

I am new in mongodb and node. I am trying to find the max value for a field (userId). But it returns nothing.
My code is
EventSchema.static("createUser",function(event,user,callback){
var That = this;
var max_usr_Id = '';
async.waterfall([
function(callback) {
That.find({"userId" : {"$ne" : ""}, "$and" : [{"userId" : {"$exists" : 1}}]}).sort({"_id" : -1}).limit(1).select("userId").exec(function(err, doc)
{
if(err)
{
console.log('User ID ERROR-');
callback({error:err,message:"Error getting max User ID"});
}else {
console.log('User ID-');
console.log(doc.userId);
max_usr_Id = doc.userId;
console.log(max_usr_Id);
}
});
console.log(max_usr_Id);
},
});
For some reason the control doesn't go inside the find function. When I try the following query in mongodb shell it works.
db.users.find({
"userId" : {
"$ne" : ""
},
"$and" : [
{
"userId" : {
"$exists" : true
}
}
]
}).sort({
"_id" : -1.0
}).limit(1);
Any help is highly appreciated. Thanks in advance.

Answer 1

The $and is not used in the proper way, try with:
That.find({ $and: [
{ "userId": { $ne: "" } },
{ "userId": { $exists: true } }
] }).sort( ...
Take a look at the $and documentation.
Edit
After seen the comments, the problem must be in the way the logging is made. You need to call toArray to get a collection, and then iterate over it (with forEach for instance):
...find( ... ).toArray(function(err, docs) {
// Print each document returned
docs.forEach(function(doc) {
console.log(doc.userId);
});
});