MongoError: FieldPath cannot be constructed with empty string

[tar-aldev]

Do you want to request a feature or report a bug?
Bug
What is the current behavior?
I am not sure if it is mongoose specific or mongodb specific.
I am getting this error

MongoError: FieldPath cannot be constructed with empty string

Which is, to be honest quite useless. It gives me no infromation about where and how it happened.
I checked my document and it does not have any empty strings at all.

If the current behavior is a bug, please provide the steps to reproduce.

1. I created a model
2. used findById() query to retrieve document with populated fields.
3. then I updated this document by mutating it's fields values (following the documentation)
4. when I call document.save() I am getting the above error.

Could it be because of .populate? (So it wants Ref not object) But why it says string then? I gues _id is a special datatype and not string.

My document with populate looks like this (some fields omitted)

...
  status: 'private',
  _id: 6082caf6fa074758d394b7e0,
  curriculumId: 5ee31a1f707484c05311d322,
  name: 'TEST 2',
  gradeFromValue: 'grade2',
  gradeToValue: 'grade3',
  teacherId: { // note this is a populated field. Can it cause this error?
    address: null,
    _id: 60811f0ca211d22586a01ec2,
    email: 'john.doe@gmail.com',
    firstName: 'John',
    lastName: 'Doe',
    access: [ [Object] ],
    createdAt: 2021-04-22T07:00:28.799Z,
    updatedAt: 2021-04-22T07:00:28.799Z,
    __v: 0,
    roles: [ 'admin' ],
    id: '60811f0ca211d22586a01ec2'
  },

As supposed, the problem is with populated fields.
.save() will preserve previously populated fields, therefore in my example I tried to populate fields that have been already populated before.

However, could it be possible to somehow improve the error message for this?
the mentioned error will be thrown.

What is the expected behavior?
I should be able to call .save or if there IS an error, it would be really useful to have a more specific message (ideally to have a field name) as now it is nearly impossible to identify what the problem is.
What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.

Node 14
"mongodb": "^3.6.5",
"mongoose": "<=5.10.18",

[Kamikadze4GAME]

@tar-aldev Please, provide script with steps to reproduce. It's too difficult to help without schema, model, query (findById().populate(...)...) etc.

You can omit some fields that don't affect the logic / algorithm.

[vkarpov15]

Without code samples or stack trace, it's unlikely we'll be able to repro this issue.

[supipd]

I have this JSON valid object (pretty print):

a = JSON.parse(`
{
    "vnut_okraj_podmienky": {
        "": {
            "standart_podmienky": {
                "type": "radio",
                "value": "on"
            },
            "nestand_teplota": {
                "type": "number",
                "value": "24"
            },
            "nestand_vlhkost": {
                "type": "number",
                "value": "70"
            }
        }
    }
}
`);

In browser console I can obtain value (=24)
a.vnut_okraj_podmienky[""].nestand_teplota.value
But trying mongosh
db.isover_projects.distinct("vnut_okraj_podmienky.''.nestand_teplota.value")
return [] ... empty list
trying mongosh
db.isover_projects.distinct("vnut_okraj_podmienky..nestand_teplota.value")
error : MongoServerError: FieldPath field names may not be empty strings.
How can I to do it without error ?

[vkarpov15]

@supipd MongoDB doesn't allow storing empty string as a key name. Nothing Mongoose can do about this unfortunately.

[TheYuriG]

Not sure if anyone is gonna get to this issue through Google, as I did, but I had this problem happening to me because I forgot a .sort('-') after my .find() and didn't realize it. That was the empty string that the error was screaming about.

[venimus]

I could also add some other case for people that would land here searching google for the same error.
I had a trailing space in my .find(...).populate(<string>), but apparently it is not related to mongoose