Array validation getting failed because of the "schema" key inside the Schema

[sourabhbagrecha]

Prerequisites

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

Mongoose version

6.2.2

Node.js version

16.14.2

MongoDB server version

4.4.6

Description

Hi Team,
I have observed a bug in the schema class/validator provided by mongoose. When a schema definition contains a key named: schema and another key containing an array of documents, the validation gets failed even with completely valid documents.

Steps to Reproduce

Step 1. Defining a schema that contains a key named schema and another key that embeds an array.

const AuthorSchema = new Schema({
  fullName: { type: "String", required: true },
});

const BookSchema = new Schema({
  schema: { type: "String", required: true },
  title: { type: "String", required: true },
  authors: [AuthorSchema],
});

const Book = model("book", BookSchema);

Step 2. Insert a document in the collection

  const book = await Book.create({
    schema: "design",
    authors: [{ fullName: "Sourabh Bagrecha" }],
    title: "The power of JavaScript",
  });

Step 3. Which throws the following error:

mongoose/node_modules/mongoose/lib/document.js:3055
    this.$__.validationError = new ValidationError(this);
                               ^

ValidationError: book validation failed: authors: Cast to Array failed for value "[ { fullName: 'Sourabh Bagrecha' } ]" (type Array) at path "authors" because of "TypeError"
    at model.Document.invalidate (/Users/sourabh/Work/repro/mongoose/node_modules/mongoose/lib/document.js:3055:32)
    ...call stack clipped...
    {
  errors: {
    authors: CastError: Cast to Array failed for value "[ { fullName: 'Sourabh Bagrecha' } ]" (type Array) at path "authors" because of "TypeError"
        at model.$set (/Users/sourabh/Work/repro/mongoose/node_modules/mongoose/lib/document.js:1417:9)
        ...call stack clipped...  
      {
      stringValue: `"[ { fullName: 'Sourabh Bagrecha' } ]"`,
      messageFormat: undefined,
      kind: 'Array',
      value: [ { fullName: 'Sourabh Bagrecha' } ],
      path: 'authors',
      reason: TypeError: doc.schema.path is not a function
          at new MongooseDocumentArray (/Users/sourabh/Work/repro/mongoose/node_modules/mongoose/lib/types/DocumentArray/index.js:60:47)
          ...call stack clipped...
      valueType: 'Array'
    }
  },
  _message: 'book validation failed'
}

Note that the validation error does not occur when:

• The field schema is removed
• The field schema is renamed to Schema (uppercase S)
• The array field is removed

The validation error occurs only when both the schema field AND the array field exist in the schema

Step 4. Now if we update the schema by uppercasing the key Schema from lowercased schema

  const BookSchema = new Schema({
    Schema: { type: "String", required: true },
// ^^^^^^^
    title: { type: "String", required: true },
    authors: [AuthorSchema],
  });

  const book = await Book.create({
    Schema: "design",
// ^^^^^^^
    authors: [{ fullName: "Sourabh Bagrecha" }],
    title: "The power of JavaScript",
  });

Step 5. It will work completely fine if schema was spelled differently (e.g. with an uppercase S, i.e.: Schema)

{
  "Schema": "design",
  "title": "The power of JavaScript",
  "authors": [
    {
      "fullName": "Sourabh Bagrecha",
      "_id": new ObjectId("633343a2396c4534d5a70e19")
    }
  ],
  "_id": new ObjectId("633343a2396c4534d5a70e18"),
  "__v": 0
}

Expected Behavior

I understand that schema is a reserved keyword that can be used to fetch the document's schema.

But in that case, it should throw a warning(compile-time) while we are specifying the schema key in our schema definition. Also, the validation is only throwing the error, when we are specifying an embedded array of documents in our definition. In all the other cases, having the schema key in our definition doesn't throw any validation error at the time of insert/update.

The expected behavior IMO would be a consistent error irrespective of the embedded array.

[IslandRhythms]

const mongoose = require('mongoose');

const {Schema} = mongoose;

const AuthorSchema = new Schema({
    fullName: { type: "String", required: true },
  });
  
  const BookSchema = new Schema({
    schema: { type: "String", required: true },
    title: { type: "String", required: true },
    authors: [AuthorSchema],
  }, {supressReservedKeysWarning: true});
  
  const Book = mongoose.model("book", BookSchema);

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

    const book = await Book.create({
        schema: "design",
        authors: [{ fullName: "Sourabh Bagrecha" }],
        title: "The power of JavaScript",
      });

      console.log('done');
  }

  run();