Describe the bug

Example schema :

{
  "name": "users",
  "properties": {
    "email": {
      "type": "string",
      "nullable": true,
      "unique": true
    },
    "firstName": {
      "type": "string"
    },
    "lastName": {
      "type": "string"
    },
    "metaData": {
      "type": "json",
      "nullable": true
    }
  },
  "indexes": [{
    "properties": "metaData.nesTed.field"
  }]
}

When calling schemaGenerator.updateSchema with that schema, the MongoSchemaGenerator raises an error

Cannot read properties of undefined (reading 'fieldNames')

It looks like the MongoSchemaGenerator.createIndexes method searches for metaData.nesTed.field key inside base properties & crashes on meta.properties[prop].fieldNames as no such property exist (which is expected, metaData being a json column).

I could fix this using options :

  "indexes": [{
    "properties": []
    "options": {
      "metaData.nesTed.field": 1
    }
  }]

However, the same fix does not work with uniques as they behave differently than pure indexes.

    "uniques": [
      {
        "properties": [],
        "options": {
          "metaData.auth.id": 1,
          "metaData.auth.provider": 1
        }
      }
    ]

This now raises :

Error in specification { name: "", unique: true, key: {} } :: caused by :: Index keys cannot be empty.

Indeed, the MongoSchemaGenerator.createUniqueIndexes only looks for target properties inside index.properties parameter, index.options parameter is only used for the 2nd MongoDB createIndex parameter.
This is different from indexes where options can be an array of two objects for :

  "indexes": [{
    "properties": [],
    "options": [{
      "metaData.auth.id": 1,
      "metaData.auth.provider": 1,
    }, {
      "unique": true
    }]
  }]

This latest example do work, but I feel like I'm twisting Mikro and that everything would be simpler if I could rely solely on the properties field.

Thanks you !

Reproduction

import { MikroORM, EntitySchema } from '@mikro-orm/mongodb';

const Users = new EntitySchema({
  name: 'Users',
  properties: {
    _id: {
      type: 'ObjectId',
      primary: true,
    },
    email: {
      type: 'string',
      nullable: true,
      unique: true,
    },
    firstName: {
      type: 'string',
    },
    lastName: {
      type: 'string',
    },
    metaData: {
      type: 'json',
      nullable: true,
    },
  },
  indexes: [
    {
      properties: 'metaData.nesTed.field',
    },
  ],
});

let orm: MikroORM;

beforeAll(async () => {
  orm = await MikroORM.init({
    entities: [Users],
    dbName: '6899',
  });
});

afterAll(async () => {
  await orm.close(true);
});

test('GH #6899', async () => {
  await orm.schema.updateSchema();
});

What driver are you using?

@mikro-orm/mongodb

MikroORM version

6.4.16

Node.js version

22

Operating system

MacOS

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• The provided reproduction is a minimal reproducible example of the bug.