clear Mongojs collections after execution

I have 2 functions which uses mongojs to connect. Both will use 2 different collections with different schemas but with same name "users". Here is the code :

var findAll = function(req, res) {
var users = db.collection('users'); //this initializes db collection
db.users.find({},function(err,users){
   if(err){
       res.json(err);
   } else{
       res.json(users);
        users = null;
   }
});
};
var getbyUserName = function(req,res) {
var username = req.params.username;    
db.users.find({'username':username},function(err,result){
    if(err){
        res.send(err);
    }else{
        res.json(result);
    }
});

};

I am not initializing "users" collection in function getbyUsername but, it is using "users" collection initialized in first function. Is there any way I can clear the collections after function 1 is successfully executed ?