Inserting into an array?

I am attempting to insert items at a specific index in an array that may or may not be empty. For example, say I have the following document in my mongoDb collection:

{
   title: "abe",
   questions: []
}

I would like to do something like set the 5th element in the array to a specific value. I was playing with the mongo db command line and I was able to do the following:

> mytest = []
[ ]
> mytest[4] = 'test'
test
> mytest
[ undefined, undefined, undefined, undefined, "test" ]

Which is basically what I want, but when I attempt to do this from my node.js code I am getting weird results (weird as in, items are not in the correct index). Code below:

Mongoose Schema Definition:

mongoose = require("mongoose")
Schema = mongoose.Schema

surveySchema = new Schema
    title: String
    questions: [
        type: Schema.Types.ObjectId
        ref: 'Question' 
    ]

Code doing the update:

surveys.update  {id: doc.survey_id}, 
{
    $push: { 
        questions: {
            $each: [doc._id],
            $position: doc.sort_order
        }
    }
}, 
{upsert:false, multi:true}, 
callback

The code above is being executed in an asynchronous loop, so it's possible the last item will be inserted before the first item and so on.

Can anyone see any reason why items would not be getting inserted at the correct index? Why does my basic example work but the code does not? Could it have to do with the fact that my schema defines questions as an array of ObjectIds?