Querying with { _id: undefined } causes "Multiple body sections in message" error

[marcj]

Description

When executing a query with undefined as a filter value (e.g., { _id: undefined }), the MongoDB driver produces a malformed wire protocol message instead of either:

• Throwing a validation error before sending the query
• Handling it gracefully (treating as "match nothing" or converting to { _id: null })

Reproduction

  import { Database } from '@deepkit/orm';
  import { MongoDatabaseAdapter } from '@deepkit/mongo';

  const database = new Database(new MongoDatabaseAdapter('mongodb://localhost:27017/test'));

  // This causes the error:
  const result = await database.query(SomeEntity)
    .filter({ _id: undefined })
    .findOneFieldOrUndefined('someField');

Error

  MongoDatabaseError: Multiple body sections in message, on connection localhost:27017 (standalone)
      at handleErrorResponse (/node_modules/@deepkit/mongo/src/client/error.ts:23:30)
      at FindCommand.handleResponse (/node_modules/@deepkit/mongo/src/client/command/command.ts:136:39)
      at MongoConnection.onResponse (/node_modules/@deepkit/mongo/src/client/connection.ts:885:34)
      at BsonStreamReader.feed (/node_modules/@deepkit/bson/src/stream.ts:67:22)
      ...

Expected Behavior

Either:

1. Validation error before sending to MongoDB: "Cannot use undefined as filter value for field '_id'"
2. Graceful handling: Treat undefined values as "no match" or strip them from the filter

Environment

• @deepkit/mongo: [version]
• @deepkit/orm: [version]
• MongoDB: 7.x / 8.x
• Node.js: 20.x / 22.x

Workaround

Guard against undefined before querying:

  if (entityId) {
    const result = await database.query(SomeEntity)
      .filter({ _id: entityId })
      .findOneFieldOrUndefined('someField');
  }

Analysis

The issue appears to be in BSON serialization - undefined values are being serialized in a way that produces an invalid MongoDB wire protocol message, causing the server to reject it with "Multiple body sections in
message".