mongoose promise and Q promise

Question

[I will answer this question but feel free to add solutions or other problems that may occur]

Working on a nodejs project where we use q promises heavily and mongoose I have found some unintuitive hickups combining the two.

First one is that mongoose promise does not have .fail or .catch.

Another one is that Model.create returns a promise that calls the resolve function with mutliple arguments when resolved. If this goes through Q then Q will call the resolve with only the first argument.

Please feel free to add more problems that may occur; I will put how I solved these things in an answer.

Answer 1

You can simply use the Q function to cast Mongoose promises to "proper" Q ones - this works in fact with every thenable.

So just use

Q(someMongoosePromise) .then(…).catch(…);

---

Multiple arguments are a different problem. However, there is a very simple hack: convert them to an array and work with that.

Q(Model.create([doc1,doc2]).then(Array)) .spread(function(d1, d2){ … });

_{(This solution doesn't apply in case Model.create returns a promise for a single number)}

Answer 2

Because mongoose promises do not have a .catch or .fail (Note that q will remove these from their library as well) you can not return a failing mongoose promise on a q stack that ends with either .catch or .fail without turning it into a q promise first. Here is how I did that (this has been answered elsewhere on stack as well):

function something(){
  return q()//make sure this function returns a promise instead of an unhandled error
  .then(function(){
    //do some stuff
  })
  .then(function(){
    return someMongoosePromise
    .then(function resolve(res){//make sure to return a q promise if this function is called in a stack that has 
     //.then.then.then(function(){return something();}).fail(...
        return q.resolve(res);
      },function reject(err){
        return q.reject(err);
      }
    );
  });
}

Second problem I encountered was that Model.create([doc1,doc2]) resolves the promise with multiple arguments but when going through q q only calls resolve with one argument:

q()
.then(function(){
  return Model.create([doc1,doc2]);
})
.then(function(docs){
  docs===doc1;//David Copperfield took doc2
});

the solution is about the same as the first one:

q()
.then(function(){
  return Model.create([doc1,doc2])
  .then(function(){//let mongoose resolve the promise
    var ret=[],i=arguments.length;
    while(--i>-1){
      ret.push(arguments[i];
    }
    return q.resolve(ret);
  });
})
.then(function(docs){
  docs===[doc1,doc2];
});