Unable to set array to `null` as default

[sebastian-nowak]

Do you want to request a feature or report a bug?

Bug in the latest mongoose version (5.2.1)

What is the current behavior?

Schemas ignore default: null for arrays, empty array is created instead.

const schema = new mongoose.Schema({
  arrayThatShouldBeNull: {
    type: [String],
    default: null,
    required: false
  },

  arrayThatShouldContainTwoEntries: {
    type: [String],
    default: ['asd', '123'],
    required: false
  }
}, {
  minimize: false // not sure if relevant
});

const Model = mongoose.model('test', schema);
const instance = new Model();

expect(instance.arrayThatShouldBeNull).to.be.null; // fails
expect(instance.arrayThatShouldContainTwoEntries).to.deep.equal(['asd', '123']); // passes

What is the expected behavior?

instance.arrayThatShouldBeNull should be set to null, but instead an empty array is created

Hi @sebastian-nowak,

if you want a null value for the array default you need to return it from a function. Your example would become:

6691.js

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

const chai = require('chai');
const expect = chai.expect;
const mongoose = require('mongoose');

const schema = new mongoose.Schema({
  arrayThatShouldBeNull: {
    type: [String],
    default: () => { return null; },
    required: false
  },

  arrayThatShouldContainTwoEntries: {
    type: [String],
    default: ['asd', '123'],
    required: false
  }
}, {
  minimize: false // not sure if relevant
});

const Model = mongoose.model('test', schema);
const instance = new Model();

expect(instance.arrayThatShouldBeNull).to.be.null; // now succeeds
expect(instance.arrayThatShouldContainTwoEntries).to.deep.equal(['asd', '123']); // passes

Output:

issues: ./6691.js
issues:

[sebastian-nowak]

Thanks for clarification. Is this behavior documented somewhere? I find it a bit surprising, I don't see any immediately obvious reason why null wouldn't work as a default value. Are there any other types that also required returning null via function?

It's mentioned here in the FAQ, but I just noticed that the example needs to be changed there as the text says null but the example returns undefined.

opened PR #6695 for this.

[shashwat-idm]

this is still a thing in mongoose v7.
Why is this the default behaviour?
If i'm explicitly setting it to null, it should be null