fix(core): initialize empty collections when fetch joining

packages/knex/src/AbstractSqlDriver.ts

@@ -115,10 +115,15 @@ export abstract class AbstractSqlDriver<C extends AbstractSqlConnection = Abstra
       // If the primary key value for the relation is null, we know we haven't joined to anything
       // and therefore we don't return any record (since all values would be null)
       const hasPK = meta2.primaryKeys.every(pk => meta2.properties[pk].fieldNames.every(name => {
-        return Utils.isDefined(root![`${relationAlias}__${name}`], true);
+        return root![`${relationAlias}__${name}`] != null;
       }));
 
       if (!hasPK) {
+        // initialize empty collections
+        if ([ReferenceType.MANY_TO_MANY, ReferenceType.ONE_TO_MANY].includes(relation.reference)) {
+          result[relation.name] = result[relation.name] || [];
+        }
+
         return;
       }
 

tests/QueryBuilder.test.ts

@@ -254,6 +254,22 @@ describe('QueryBuilder', () => {
     await qb.execute();
   });
 
+  test('join and select with empty collections', async () => {
+    const qb = orm.em.createQueryBuilder(FooBar2, 'fb');
+    qb.select('*')
+      .leftJoinAndSelect('fb.tests', 't')
+      .orderBy({ name: 1 });
+
+    await orm.em.nativeInsert(Test2, { id: 1, name: 't' });
+    await orm.em.nativeInsert(FooBar2, { id: 1, name: 'fb 1', tests: [] });
+    await orm.em.nativeInsert(FooBar2, { id: 2, name: 'fb 2', tests: [1] });
+    const res = await qb.getResultList();
+    expect(res[0].tests.isInitialized()).toBe(true);
+    expect(res[0].tests.getItems()).toHaveLength(0);
+    expect(res[1].tests.isInitialized()).toBe(true);
+    expect(res[1].tests.getItems()).toHaveLength(1);
+  });
+
   test('select leftJoin 1:1 inverse', async () => {
     const qb = orm.em.createQueryBuilder(FooBaz2, 'fz');
     qb.select(['fb.*', 'fz.*'])