possible bug: SchemaTypes.ObjectId.cast does not run when value is `null`

So, I came across a case where I want to set all object ids to be undefined if the value passed to document is null. The reason being that I want to avoid having null keys in my documents, and prefer not having them altogether.

However, trying to do that via custom casting logic did not work because the cast function is not invoked when the value is null.

'use strict';
disallowNullInObjectId();

// does not work
function disallowNullInObjectId () {
  const originalObjectIdCast = mongoose.SchemaTypes.ObjectId.cast();

  mongoose.SchemaTypes.ObjectId.cast(function (value) {
    if (value === null) {
      return undefined;
    }
    return originalObjectIdCast(value);
  });
}



const userSchema = new Schema({
  name: { type: String },
  cityId: Schema.ObjectId
});

const User = mongoose.model('User', userSchema);

const user = new User({ name: 'Hafez', cityId: null });

// fails, user.cityId is `null` instead of `undefined`
assert.strictEqual(user.cityId, undefined);

I was able to work around this by setting a default set value to all object ids.

// works
function disallowNullInObjectId () {
  mongoose.SchemaTypes.ObjectId.set('set', (value) => {
    if (value === null) {
      return undefined;
    }

    return value;
  });
}

[vkarpov15]

We explicitly avoid casting nullish values with this line: https://github.com/Automattic/mongoose/blob/b9dd74b3a9f9b9237d40a16fe6f4b28da9ee60fb/lib/schematype.js#L1105-L1107 . Whether we continue doing this is something worth discussing. I think the general idea is that cast functions shouldn’t always have to write if (v == null) { return v }, but I can see arguments on both sides. What do you think @AbdelrahmanHafez ?

The current workaround is to use a setter as you described.

[vkarpov15]

Yeah, global schematype validation: https://mongoosejs.com/docs/validation.html#global-schematype-validation mongoose.Schema.ObjectId.set('validate', v => v !== null)