How to have an auto calculated attribute in mongoose schema

Question

suppose I have following mongoose schema in Node.js

var schema = new mongoose.Schema({
    views:Number,
    clicks:Number,
    ctr:Number
})

In which I want that CTR attribute should be automatically calculated by a formula based on values of other attributes, i.e lets say

ctr = clicks / views

I don't want to have CTR as a virtual field because based on the value of CTR I will sort all documents of my collection and apparently sorting is not possible based on virtual fields. So I was wondering if there is any way to have calculated attributes in mongoose schema which gets automatically updated on update of any of the dependent attribute/field

Answer 1

I want that CTR attribute should be automatically calculated by a formula based on values of other attributes

From the Mongoose docs [emphasis added]:

You can also set the default schema option to a function. Mongoose will execute that function and use the return value as the default.

Therefore, your example could be rewritten as:

const schema = new mongoose.Schema({
  views: {
    type: Number
  },
  clicks: {
    type: Number
  },
  ctr: {
    type: Number,
    default: function() {
      return this.clicks / this.views
    }
  }
})

So I was wondering if there is any way to have calculated attributes in mongoose schema which gets automatically updated on update of any of the dependent attribute/field

If I understand correctly, note the section in the docs on the setDefaultsOnInsert option. This may be of relevance when recalculating the default upon update.

Answer 2

You can use for it virtual field:

var schema = new mongoose.Schema({
    views:Number,
    clicks:Number
})

schema.virtual('ctr')
  .get(function() {
    return this.clicks / this.views;
  });

Answer 3

You can do anything, but don't forget about that that difficult thing slower your server.

In my case for calculating age user I do that.

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

    const userSchema = new Schema( {

      born: {
        type: String,
        get: a => {
          let date = new Date(a);
          let today = new Date();
          let timeDiff = today.getTime() - date.getTime();
          let age = Math.floor( timeDiff / ( 1000 * 3600 * 24 ) / 365 );
          return age;
        },
        set: age => age,
        alias: 'age',
      },
    } 

);

module.exports = mongoose.model( 'User', userSchema );

And Mongoose Documantation