How to define a sort function in Mongoose

Question

I'm developing a small NodeJS web app using Mongoose to access my MongoDB database. A simplified schema of my collection is given below:

var MySchema = mongoose.Schema({                                 
    content:   { type: String },     
    location:  {                                                        
         lat:      { type: Number },                       
         lng:      { type: Number },                                              
    },
    modifierValue:  { type: Number }     
});

Unfortunately, I'm not able to sort the retrieved data from the server the way it is more convenient for me. I wish to sort my results according to their distance from a given position (location) but taking into account a modifier function with a modifierValue that is also considered as an input.

What I intend to do is written below. However, this sort of sort functionality seems to not exist.

MySchema.find({})
        .sort( modifierFunction(location,this.location,this.modifierValue) )
        .limit(20)       // I only want the 20 "closest" documents
        .exec(callback)

The mondifierFunction returns a Double.

So far, I've studied the possibility of using mongoose's $near function, but this doesn't seem to sort, not allow for a modifier function.

Since I'm fairly new to node.js and mongoose, I may be taking a completely wrong approach to my problem, so I'm open to complete redesigns of my programming logic.

Thank you in advance,

Answer 1

You might have found an answer to this already given the question date, but I'll answer anyway.

For more advanced sorting algorithms you can do the sorting in the exec callback. For example

MySchema.find({})
  .limit(20)
  .exec(function(err, instances) {
      let sorted = mySort(instances); // Sorting here

      // Boilerplate output that has nothing to do with the sorting.
      let response = { };

      if (err) {
          response = handleError(err);
      } else {
          response.status = HttpStatus.OK;
          response.message = sorted;
      }

      res.status(response.status).json(response.message);
  })

mySort() has the found array from the query execution as input and the sorted array as output. It could for instance be something like this

function mySort (array) {
  array.sort(function (a, b) {
    let distanceA = Math.sqrt(a.location.lat**2 + a.location.lng**2);
    let distanceB = Math.sqrt(b.location.lat**2 + b.location.lng**2);

    if (distanceA < distanceB) {
      return -1;
    } else if (distanceA > distanceB) {
      return 1;
    } else {
      return 0;
    }
  })

  return array;
}

This sorting algorithm is just an illustration of how sorting could be done. You would of course have to write the proper algorithm yourself. Remember that the result of the query is an array that you can manipulate as you want. array.sort() is your friend. You can information about it here.