"is not a function" validation error with nested Schema

[Sharlaan]

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

Possibly related to issue #8190

What is the current behavior?

const CategorySchema = new Schema(
  {
    name: String,
    // categoryUrl: String, // WORKS
    categoryUrl: { type: String, required: true }, // FAILS
    isSelected: Boolean,
  },
  { _id: false },
);

const SiteSchema = new Schema(
  {
    categoryUrls: [CategorySchema],
  },
  {
    toJSON: {
      transform(doc, ret) {
        ret.id = ret._id;
        delete ret._id;
        delete ret.__v;
      },
    },
  },
);

try {
  const newSite = await Site.create(request.body);
  return new HttpResponseCreated({ id: newSite.id });
} catch (error) {
  return new HttpResponseInternalServerError(
    `[SiteController] createSite failed with error ${error.message}`,
  );
}

returns validation error while trying to save a new doc (properly formatted) in DB :
[SiteController] createSite failed with error Site validation failed: categoryUrls.0.categoryUrl: this.isSelected is not a function

What is the expected behavior?
should save new docs properly.

It took me quite some time to figure it werenot the 'isSelected' property making mongoose fail, but the property just above and its required check ...
Not sure if this could be qualified as a "bug", but the error message is really misleading ...
How should i make this nested propertty categoryUrl as required without it making the whole Schema validation fail ?

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
NodeJS: 14.11.0
Mongoose: 5.10.6
MongoDB Server: 4.4.1

[AbdelrahmanHafez]

9438.js

'use strict';
const mongoose = require('mongoose');
const { Schema } = mongoose;
const assert = require('assert');

const categorySchema = new Schema(
  {
    name: String,
    // categoryUrl: String, // Makes tests pass
    categoryUrl: { type: String, required: true }, // Makes test fail
    isSelected: Boolean
  },
  { _id: false }
);

const siteSchema = new Schema({ categoryUrls: [categorySchema] });

const Site = mongoose.model('Site', siteSchema);
const site = new Site({
  categoryUrls: [
    { name: 'A', categoryUrl: 'B', isSelected: false }
  ]
});
const err = site.validateSync();
console.log(err);

assert.ok(err == null);
console.log('All assertions passed.');

Output

    'categoryUrls.0.categoryUrl': ValidatorError: Path `categoryUrl` is required.
        at validate (\mongoose\lib\schematype.js:1358:13)
        at \mongoose\lib\schematype.js:1347:5
        at Array.forEach (<anonymous>)
        at SchemaString.SchemaType.doValidateSync (\mongoose\lib\schematype.js:1301:14)
        at \mongoose\lib\document.js:2534:19
        at Array.forEach (<anonymous>)
        at EmbeddedDocument.Document.validateSync (\mongoose\lib\document.js:2518:9)
        at DocumentArrayPath.doValidateSync (\mongoose\lib\schema\documentarray.js:302:37)
        at \mongoose\lib\document.js:2534:19
        at Array.forEach (<anonymous>)
        at model.Document.validateSync (\mongoose\lib\document.js:2518:9)
        at Object.<anonymous> (test.js:24:18)
        at Module._compile (internal/modules/cjs/loader.js:1076:30)
        at Object.Module._extensions..js (internal/modules/cjs/loader.js:1097:10)
        at Module.load (internal/modules/cjs/loader.js:941:32)
        at Function.Module._load (internal/modules/cjs/loader.js:782:14) {
      properties: [Object],
      kind: 'required',
      path: 'categoryUrl',
      value: 'B',
      reason: TypeError: this.isSelected is not a function
          at EmbeddedDocument.requiredValidator (\mongoose\lib\schematype.js:918:41)
          at \mongoose\lib\schematype.js:1335:24
          at Array.forEach (<anonymous>)
          at SchemaString.SchemaType.doValidateSync (\mongoose\lib\schematype.js:1301:14)
          at \mongoose\lib\document.js:2534:19
          at Array.forEach (<anonymous>)
          at EmbeddedDocument.Document.validateSync (\mongoose\lib\document.js:2518:9)
          at DocumentArrayPath.doValidateSync (\mongoose\lib\schema\documentarray.js:302:37)
          at \mongoose\lib\document.js:2534:19
          at Array.forEach (<anonymous>)
          at model.Document.validateSync (\mongoose\lib\document.js:2518:9)
          at Object.<anonymous> (test.js:24:18)
          at Module._compile (internal/modules/cjs/loader.js:1076:30)
          at Object.Module._extensions..js (internal/modules/cjs/loader.js:1097:10)
          at Module.load (internal/modules/cjs/loader.js:941:32)
          at Function.Module._load (internal/modules/cjs/loader.js:782:14),
      [Symbol(mongoose:validatorError)]: true
    }
  },
  _message: 'Site validation failed'
}

Seems like mongoose uses an internal function isSelected, and is failing due to the fact that we're using a field isSelected in the subschema.