validation ignored if the path set as 'select: false'

[ryanmz1]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the issue has not already been raised

Issue

Related Issue:
#730
[https://github.com//issues/730](issue 730)

...
const User = mongoose.model('User', Schema({
  name: { type: String, required: true },
  age: { type: Number, required: true, **select:false** }
});
// set user's name and age
// user={name:'Ben',age:16}
...
// then update and save in this way
const user=await User.findOne({name}); // user={name:'Ben'}
// do some work
...
await user.save(); // does not run validation on 'age'
...

At most time, before doc.save(), validation will run. But if select option set to false on some path, it does not run validation on that path even there's a required validator until select set to true. This idea may come from #730. That's reasonable enough.

But it's so sad that this rationale cannot be searched on the documentation. I only found it in source code, around line 2610:

I just thought full validation will run rather than select option determines. It wasted some time.
So, for making more lives much easier, I think this rule should be added to the documentation directly and explicitly, at least a warning item of rules on validation ;)

[IslandRhythms]

The following script throws an error as expected. Please modify the below script to demonstrate the issue you're seeing.

const mongoose = require('mongoose');

const testSchema = new mongoose.Schema({
  name: { type: String, required: true },
  age: { type: Number, required: true, select: false }
});

const Test = mongoose.model('Test', testSchema);

async function run() {
  await mongoose.connect('mongodb://localhost:27017');
  await mongoose.connection.dropDatabase();

  await Test.create({
    name: 'Test Testerson',
    age: 0
  });

  const doc = await Test.findOne();
  doc.age = null;
  doc.name = 'Test';
  await doc.save();
  console.log('done');
}

run();

[ryanmz1]

const mongoose = require('mongoose');

const testSchema = new mongoose.Schema({
  name: { type: String, required: true },
  age: { type: Number, required: true, select: false },
  links: { type: String, required: true, select: false }
});

testSchema.pre('save', function() {
  **this.links = undefined;**
});

const Test = mongoose.model('Test', testSchema);

async function run() {
  await mongoose.connect('mongodb://localhost:27017');
  await mongoose.connection.dropDatabase();

  await Test.create({
    name: 'Test Testerson',
    age: 0,
    **links: 'some init links'**
  });

  const doc = await Test.findOne(); // error occur: .select('+links');
  
  doc.name = 'Test';
  **await doc.save();**
  console.log('done');
}
run();

"links" set to undefined manually in pre save middleware, but finally there is no error. When you turn on select option, then error occur as expected.

Maybe it's a very weird usage situation above but actually the most interesting thing I see are just:

1. validation happens at a very first time among pre save middlewares.
2. the select option does determine validation paths list, which is difficult for beginners to realize though.

The following script throws an error as expected. Please modify the below script to demonstrate the issue you're seeing.

const mongoose = require('mongoose');

const testSchema = new mongoose.Schema({
  name: { type: String, required: true },
  age: { type: Number, required: true, select: false }
});

const Test = mongoose.model('Test', testSchema);

async function run() {
  await mongoose.connect('mongodb://localhost:27017');
  await mongoose.connection.dropDatabase();

  await Test.create({
    name: 'Test Testerson',
    age: 0
  });

  const doc = await Test.findOne();
  doc.age = null;
  doc.name = 'Test';
  await doc.save();
  console.log('done');
}

run();

[IslandRhythms]

const mongoose = require('mongoose');

const testSchema = new mongoose.Schema({
  name: { type: String, required: true },
  age: { type: Number, required: true, select: false },
  links: { type: String, required: true, select: false }
});

testSchema.pre('save', function() {
  this.links = undefined;
});

const Test = mongoose.model('Test', testSchema);

async function run() {
  await mongoose.connect('mongodb://localhost:27017');
  await mongoose.connection.dropDatabase();

  await Test.create({
    name: 'Test Testerson',
    age: 0,
    links: 'some init links'
  });

  const doc = await Test.findOne(); // error occur: .select('+links');
  
  doc.name = 'Test';
  await doc.save();
  console.log('what is doc', doc);
  console.log('done');
}
run();