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Mongoose, sort query by populated field

Posted by: admin November 30, 2017 Leave a comment

Questions:

As far as I know, it’s possible to sort populated docs with Mongoose (source).

I’m searching for a way to sort a query by one or more populated fields.

Consider this two Mongoose schemas :

var Wizard = new Schema({
    name  : { type: String }
, spells  : { [{ type: Schema.ObjectId, ref: 'Spell' }] }
});

var Spell = new Schema({
    name    : { type: String }
,   damages : { type: Number }
});

Sample JSON:

[{
    name: 'Gandalf',
    spells: [{
            name: 'Fireball',
            damages: 20
        }]
}, {
    name: 'Saruman',
    spells: [{
            name: 'Frozenball',
            damages: 10
        }]
}, {
    name: 'Radagast',
    spells: [{
            name: 'Lightball',
            damages: 15
        }]
}]

I would like to sort those wizards by their spell damages, using something like :

WizardModel
  .find({})
  .populate('spells', myfields, myconditions, { sort: [['damages', 'asc']] })
// Should return in the right order: Saruman, Radagast, Gandalf

I’m actually doing those sorts by hands after querying and would like to optimize that.

Answers:

Ask yourself(and here are the answers):

What I want?
Sort wizards by their spell damage(it should be an aditional field, probably the sum of the spells damage.

What I did:
I have sorted the SPELLS of the wizard.

What should you do:
Wizard.find({}).sort({ power : ‘asc’ }) and then populate with spells and do whatever you like.
Power is another field in Wizard. You will need it because even if you populate your spells, you will have an array of spells and it won’t help you.

Hope this helps.

Questions:
Answers:

You can implicitly specify only required parameters of populate method:

WizardModel
  .find({})
  .populate({path: 'spells', options: { sort: [['damages', 'asc']] }})

Have a look at http://mongoosejs.com/docs/api.html#document_Document-populate
Here is an example from a link above.

doc
.populate('company')
.populate({
  path: 'notes',
  match: /airline/,
  select: 'text',
  model: 'modelName'
  options: opts
}, function (err, user) {
  assert(doc._id == user._id) // the document itself is passed
})

Questions:
Answers:

Even though this is rather an old post, I’d like to share a solution through the MongoDB aggregation lookup pipeline

The important part is this:

 {
      $lookup: {
        from: 'spells',
        localField: 'spells',
        foreignField:'_id',
        as: 'spells'
      }
    },
    {
      $project: {
        _id: 1,
        name: 1,
        // project the values from damages in the spells array in a new array called damages
        damages: '$spells.damages',
        spells: {
          name: 1,
          damages: 1
        }
      }
    },
    // take the maximum damage from the damages array
    {
      $project: {
        _id: 1,
        spells: 1,
        name: 1,
        maxDamage: {$max: '$damages'}
      }
    },
    // do the sorting
    {
      $sort: {'maxDamage' : -1}
    }

Find below a complete example

'use strict';

const mongoose = require('mongoose');
const Schema = mongoose.Schema;

mongoose.connect('mongodb://localhost/lotr');

const db = mongoose.connection;

db.on('error', console.error.bind(console, 'connection error:'));
db.once('open', () => {



  let SpellSchema = new Schema({
    name    : { type: String },
    damages : { type: Number }
  });

  let Spell = mongoose.model('Spell', SpellSchema);

  let WizardSchema = new Schema({
    name: { type: String },
    spells: [{ type: Schema.Types.ObjectId, ref: 'Spell' }]
  });

  let Wizard = mongoose.model('Wizard', WizardSchema);

  let fireball = new Spell({
    name: 'Fireball',
    damages: 20
  });

  let frozenball = new Spell({
    name: 'Frozenball',
    damages: 10
  });

  let lightball = new Spell({
    name: 'Lightball',
    damages: 15
  });

  let spells = [fireball, frozenball, lightball];

  let wizards = [{
    name: 'Gandalf',
    spells:[fireball]
  }, {

    name: 'Saruman',
    spells:[frozenball]
  }, {
    name: 'Radagast',
    spells:[lightball]
  }];

  let aggregation = [
    {
      $match: {}
    },
    // find all spells in the spells collection related to wizards and fill populate into wizards.spells
    {
      $lookup: {
        from: 'spells',
        localField: 'spells',
        foreignField:'_id',
        as: 'spells'
      }
    },
    {
      $project: {
        _id: 1,
        name: 1,
        // project the values from damages in the spells array in a new array called damages
        damages: '$spells.damages',
        spells: {
          name: 1,
          damages: 1
        }
      }
    },
    // take the maximum damage from the damages array
    {
      $project: {
        _id: 1,
        spells: 1,
        name: 1,
        maxDamage: {$max: '$damages'}
      }
    },
    // do the sorting
    {
      $sort: {'maxDamage' : -1}
    }
  ];
  Spell.create(spells, (err, spells) => {
    if (err) throw(err);
    else {
      Wizard.create(wizards, (err, wizards) =>{
        if (err) throw(err);
        else {
          Wizard.aggregate(aggregation)
          .exec((err, models) => {
            if (err) throw(err);
            else {
              console.log(models[0]); // eslint-disable-line
              console.log(models[1]); // eslint-disable-line
              console.log(models[2]); // eslint-disable-line
              Wizard.remove().exec(() => {
                Spell.remove().exec(() => {
                  process.exit(0);
                });
              });
            }
          });
        }
      });
    }
  });
});