fix(mongo): move $fulltext from $and to top level (#4066)

Closes #4065

---------

Co-authored-by: Martin Adámek <banan23@gmail.com>

packages/mongodb/src/MongoDriver.ts

@@ -138,14 +138,29 @@ export class MongoDriver extends DatabaseDriver<MongoConnection> {
 
   private renameFields<T>(entityName: string, data: T, where = false): T {
     // copy to new variable to prevent changing the T type or doing as unknown casts
-    const copiedData: T & { $fulltext?: string; $text?: { $search: string } } = Object.assign({}, data); // copy first
+    const copiedData: Dictionary = Object.assign({}, data); // copy first
     Utils.renameKey(copiedData, 'id', '_id');
     const meta = this.metadata.find(entityName);
 
     if (meta) {
       this.inlineEmbeddables(meta, copiedData, where);
     }
 
+    // If we had a query with $fulltext and some filter we end up with $and with $fulltext in it.
+    // We will try to move $fulltext to top level.
+    if (copiedData.$and) {
+      for (let i = 0; i < copiedData.$and.length; i++) {
+        const and = copiedData.$and[i];
+        if ('$fulltext' in and) {
+          if ('$fulltext' in copiedData) {
+            throw new Error('Cannot merge multiple $fulltext conditions to top level of the query object.');
+          }
+          copiedData.$fulltext = and.$fulltext!;
+          delete and.$fulltext;
+        }
+      }
+    }
+
     // move search terms from data['$fulltext'] to mongo's structure: data['$text']['search']
     if ('$fulltext' in copiedData) {
       copiedData.$text = { $search: copiedData.$fulltext! };

tests/issues/GH4065.test.ts

@@ -0,0 +1,29 @@
+import { MikroORM } from '@mikro-orm/mongodb';
+
+import { Author, Book } from '../entities';
+import { closeReplSets, initORMMongo } from '../bootstrap';
+
+describe('GH4065', () => {
+
+  let orm: MikroORM;
+
+  beforeAll(async () => orm = await initORMMongo(true));
+  beforeEach(async () => orm.schema.clearDatabase());
+
+  afterAll(async () => {
+    await orm.close();
+    await closeReplSets();
+  });
+
+  test('should load entities with $fulltext and filter set', async () => {
+    const god = new Author('God', 'hello@heaven.god');
+    const bible = new Book('Bible', god);
+    await orm.em.persistAndFlush(bible);
+    orm.em.clear();
+
+    const booksRepository = orm.em.getRepository(Book);
+    orm.em.addFilter('BreakFulltext', { title: { $re: '.*' } }, Book, true);
+    const books = await booksRepository.find({ $fulltext: 'Bible' });
+    expect(books.length).toBe(1);
+  });
+});