Return data from mongoDB nested queries

What is the method to return data from nested queries.

Im merging multiple function calls into one single module , I have the following code

  retrieveStudentSessions: function(req, res, callback){
     MongoClient.connect(config.mongoPath+config.dbName, function(err, db) {
       if(err){
         return callback(new Error("Unable to Connect to DB"));
       }
      var collection        = db.collection(config.userList);
      var sessionCollection = db.collection(config.session);
      var templateCollection = db.collection(config.template);
      var tempSession  = {session:[], templates:[]};

      //Obtain UserID
      collection.find({'email' : req.user['_json'].email}).nextObject(function(err, doc) {
          if(err){
                return callback(new Error("Error finding user in DB"));
          } 
          //Search Session collection for userID
            sessionCollection.find({ $or : [{'studentsB' : doc['userid']},{'studentsA' : doc['userid']}]}).each(function(err, doc) {
              if(err){
                  return callback(new Error("Error finding user in DB"));
              }  
                  //Update JSON
                  tempSession.session.push(doc);
                  //Query for Template Title using Template ID from Session Collection
                  templateCollection.find({'_id' : doc['templateId']}).nextObject(function(err, doc){
                    if(err){
                          return callback(new Error("Error finding user in DB"));
                    } 
                    //Update JSON
                    tempSession.templates.push(doc);
                  });
            });
        return callback(null, tempSession);
      });
    });
  }

Caller Function

dbCall.retrieveStudentSessions(req, res, function(err, result){
                if(err){
                    console.log(err);
                    return;
                }
                console.log(result);
            });

Above code returns error undefined is not a function when i try to return variable tempSession. The same works fine with single queries . Is there any specific method of return, when it comes to nested queries?