fix(core): reset collections when assigning to those not initialized

Previously when one assigned some collection values to not initialized collection, it would
not wipe it when flushing, keeping the old values there.

packages/core/src/entity/Collection.ts

@@ -8,7 +8,7 @@ import { wrap } from './wrap';
 
 export class Collection<T extends AnyEntity<T>, O extends AnyEntity<O> = AnyEntity> extends ArrayCollection<T, O> {
 
-  private snapshot: T[] = []; // used to create a diff of the collection at commit time
+  private snapshot: T[] | undefined = []; // used to create a diff of the collection at commit time
   private initialized = false;
   private dirty = false;
   private _populated = false;
@@ -66,10 +66,13 @@ export class Collection<T extends AnyEntity<T>, O extends AnyEntity<O> = AnyEnti
       this.validateModification(items);
     }
 
+    const wasInitialized = this.initialized;
     this.initialized = true;
     super.hydrate(items);
 
-    if (takeSnapshot) {
+    if (!wasInitialized) {
+      this.snapshot = undefined;
+    } else if (takeSnapshot) {
       this.takeSnapshot();
     }
   }

packages/knex/src/AbstractSqlDriver.ts

@@ -231,15 +231,16 @@ export abstract class AbstractSqlDriver<C extends AbstractSqlConnection = Abstra
     const wrapped = wrap(coll.owner, true);
     const meta = wrapped.__meta;
     const pks = wrapped.__primaryKeys;
-    const snapshot = coll.getSnapshot().map(item => wrap(item, true).__primaryKeys);
+    const snap = coll.getSnapshot();
+    const snapshot = snap ? snap.map(item => wrap(item, true).__primaryKeys) : [];
     const current = coll.getItems(false).map(item => wrap(item, true).__primaryKeys);
-    const deleteDiff = snapshot.filter(item => !current.includes(item));
+    const deleteDiff = snap ? snapshot.filter(item => !current.includes(item)) : true;
     const insertDiff = current.filter(item => !snapshot.includes(item));
     const target = snapshot.filter(item => current.includes(item)).concat(...insertDiff);
     const equals = Utils.equals(current, target);
 
     // wrong order if we just delete and insert to the end
-    if (coll.property.fixedOrder && !equals) {
+    if (coll.property.fixedOrder && !equals && Array.isArray(deleteDiff)) {
       deleteDiff.length = insertDiff.length = 0;
       deleteDiff.push(...snapshot);
       insertDiff.push(...current);

tests/EntityHelper.mysql.test.ts

@@ -79,6 +79,27 @@ describe('EntityHelperMySql', () => {
     expect(book2.tags.getIdentifiers()).toMatchObject([tag2.id]);
   });
 
+  test('assign() should update not initialized collection [mysql]', async () => {
+    const other = new BookTag2('other');
+    await orm.em.persistAndFlush(other);
+    const jon = new Author2('Jon Snow', 'snow@wall.st');
+    const book = new Book2('Book2', jon);
+    const tag1 = new BookTag2('tag 1');
+    const tag2 = new BookTag2('tag 2');
+    const tag3 = new BookTag2('tag 3');
+    book.tags.add(tag1, tag2, tag3);
+    await orm.em.persistAndFlush(book);
+    orm.em.clear();
+
+    const book1 = await orm.em.findOneOrFail(Book2, book.uuid);
+    wrap(book1).assign({ tags: [tag1.id, other.id] });
+    await orm.em.flush();
+    orm.em.clear();
+
+    const book2 = await orm.em.findOneOrFail(Book2, book.uuid, ['tags']);
+    expect(book2.tags.getIdentifiers()).toMatchObject([tag1.id, other.id]);
+  });
+
   test('assign() allows deep merging of object properties [mysql]', async () => {
     const jon = new Author2('Jon Snow', 'snow@wall.st');
     const book = new Book2('Book2', jon);