Stubbing virtual attributes of Mongoose model - node.js

Question

Is there a way to stub a virtual attribute of a Mongoose Model?
Assume Problem is a model class, and difficulty is a virtual attribute. delete Problem.prototype.difficulty returns false, and the attribute is still there, so I can't replace it with any value I want.
I also tried
var p = new Problem();
delete p.difficulty;
p.difficulty = Problem.INT_EASY;
It didn't work.
Assigning undefined to Problem.prototype.difficulty or using sinon.stub(Problem.prototype, 'difficulty').returns(Problem.INT_EASY);
would throw an exception "TypeError: Cannot read property 'scope' of undefined", while doing
var p = new Problem();
sinon.stub(p, 'difficulty').returns(Problem.INT_EASY);
would throw an error "TypeError: Attempted to wrap string property difficulty as function".
I am running out of ideas. Help me out! Thanks!

Answer 1

mongoose internally uses Object.defineProperty for all properties. Since they are defined as non-configurable, you can't delete them, and you can't re-configure them, either.
What you can do, though, is overwriting the model’s get and set methods, which are used to get and set any property:
var p = new Problem();
p.get = function (path, type) {
if (path === 'difficulty') {
return Problem.INT_EASY;
}
return Problem.prototype.get.apply(this, arguments);
};
Or, a complete example using sinon.js:
var mongoose = require('mongoose');
var sinon = require('sinon');
var problemSchema = new mongoose.Schema({});
problemSchema.virtual('difficulty').get(function () {
return Problem.INT_HARD;
});
var Problem = mongoose.model('Problem', problemSchema);
Problem.INT_EASY = 1;
Problem.INT_HARD = 2;
var p = new Problem();
console.log(p.difficulty);
sinon.stub(p, 'get').withArgs('difficulty').returns(Problem.INT_EASY);
console.log(p.difficulty);

Answer 2

As of the end of 2017 and the current Sinon version, stubbing only some of the arguments (e.g. only virtuals on mongoose models) can be achieved in the following manner
const ingr = new Model.ingredientModel({
productId: new ObjectID(),
});
// memorizing the original function + binding to the context
const getOrigin = ingr.get.bind(ingr);
const getStub = S.stub(ingr, 'get').callsFake(key => {
// stubbing ingr.$product virtual
if (key === '$product') {
return { nutrition: productNutrition };
}
// stubbing ingr.qty
else if (key === 'qty') {
return { numericAmount: 0.5 };
}
// otherwise return the original
else {
return getOrigin(key);
}
});
The solution is inspired by a bunch of different advises, including one by #Adrian Heine