Method not found Error

I am new to node.js and mongo db. What I am trying to do is to call the ordered function in my model.js from index.js but I have a complain

object function model() has no method ordered()

routes/index.js

var pics_ = models.Picture.ordered();

model.js

Picture.prototype.ordered = function() {
var ordered = mongoose.Picture.find().sort({points:-1}).toArray()
console.log(ordered);
return ordered;
};