Mongoose: Documentation: Difference between adding keys to schemas vs updating schema

[govindrai]

In the mongoose docs, under the Defining Schemas section the docs state:

If you want to add additional keys later, use the Schema#add method.

I want to understand: Is there a difference between using Schema.prototype.add() and simply updating the schema?

[lineus]

@govindrai the instance of Schema that is returned by new Schema has several properties that are set based on the properties of the object passed into new Schema. The two most relevant to your question are paths, and tree. The add command basically calls the path method on each path/nested path which updates the paths and tree properties of the schema.

Here is an example that will hopefully clarify this:

note that the something path is not set in the saved document

6533.js

#!/usr/bin/env node
'use strict';

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/test');
const conn = mongoose.connection;
const Schema = mongoose.Schema;

const schema = new Schema({
  name: String
});

schema.something = Number; // is this what you meant by updating the schema?

schema.add({ somethingElse: Number });

schema.path('thirdThing', { type: Number });

const Test = mongoose.model('test', schema);

const test = new Test({
  name: 'Billy',
  something: 34,
  somethingElse: 23,
  thirdThing: 12
});

async function run() {
  await conn.dropDatabase();
  await test.save();
  let found = await Test.findOne({});
  console.log(found);
  return conn.close();
}

run();

Output:

issues: ./6533.js
{ _id: 5b0d37184ebd4721d5fcc7a8,
  name: 'Billy',
  somethingElse: 23,
  thirdThing: 12,
  __v: 0 }
issues:

Please let me know if I misunderstood your question :smile: . I hope this helps.

[vkarpov15]

@govindrai what do you mean by "simply updating the schema"? If you do new Schema({ foo: String, bar: String }) and you change it to new Schema({ foo: String, bar: String, baz: String }), there's no difference between that and new Schema({ foo: String, bar: String }).add({ baz: String }) as far as mongoose is concerned.