MongooseDocument { undefined } and doc.someProp

[thernstig]

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

Feature? Bug?

What is the current behavior?

I have a document field like:

var parentSchema = new Schema({
  time: Date,
  child:  {
    prop1: String,
    prop2: String,
  }
}

According to https://mongoosejs.com/docs/subdocs.html#altsyntaxsingle this should not work since I do not have { typePojoToMixed: false }.

Also, in the example in that link above it also shows that type: is needed (is that correct that it is needed?)

But it does work in a sense, or I am confused on what is supposed to happen according to the docs that says it is a mixed type instead.

I query (findOne) and get a document that has only filled in the date. When I then do if (doc.child) it returns true. Checking console.log(doc.child) gives MongooseDocument { undefined }.

Is the intention here that I need to do doc.child.toObject() (or lean() on the query) to have this work? Or is there a way mongoose could instead return undefined subdocs as undefined instead?

The reason I am asking is that this caused a bug in the above if check where the subdoc was undefined, but returned truthy since it was gave MongooseDocument { undefined }.

What is the expected behavior?

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.

Mongoose 5.9.13
Node.js 12.15.0
MongoDB 3.6

[vkarpov15]

You don't need type, that schema declaration works as expected and is not a mixed type. We'll improve the documentation around this.

The reason why doc.child is not undefined is explained in this comment: #5310 (comment) . TLDR; Mongoose needs it for change tracking on nested paths, because you can't Object.defineProperty() on a null value.

2 ways to work around this:

1. Check if (doc.child.$isEmpty()) rather than if (doc.child == null)
2. Use a single nested subdoc instead: new Schema({ child: new Schema({ prop1: String }) })

[thernstig]

@vkarpov15 Does this mean if I remove minimize: false on the schema I have, then if (doc.child) will work in this case?

I read here https://mongoosejs.com/docs/guide.html#minimize that doc.inventory; // {} when minimize: false, but should that piece of text instead show MongooseDocument { undefined }?

[thernstig]

Also if (doc.child.$isEmpty()) did not work, it still passes as truthy. To be more explicit, this is what I see:

  console.log(doc.child);              // MongooseDocument { undefined }
  console.log(doc.child.$isEmpty());   // true
  console.log(doc.child.toObject());   // undefined

[vkarpov15]

minimize: false just means that Mongoose won't automatically remove empty objects. if (doc.child) will still always be true.

Re: doc.inventory; // {}, you're right, we should change that.

Re: checking for empty, you should do if (!doc.child.$isEmpty()). $isEmpty() returns false if you have an empty nested path.

[thernstig]

Thanks. The MongooseDocument { undefined } is a bit confusing (and unconventional) hence, so easy to miss one has to use $isEmpty() - haven't seen other projects use that for empty objects but I do see the reasons being mentioned here #5310 (comment) - but I bet this will always continue to be a source of confusion for users, considering a nested path looks exactly like a nested object on a prop.

[vkarpov15]

I agree it is confusing and unconventional. We would love to replace this behavior someday, but don't have a good idea of how to do so right now. I'm hoping that the work we're doing with proxies for #8884 will lead to a better solution.