diff --git a/packages/core/src/entity/Collection.ts b/packages/core/src/entity/Collection.ts
index de74f9df1693..4ff47cab8c61 100644
--- a/packages/core/src/entity/Collection.ts
+++ b/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();
     }
   }
diff --git a/packages/knex/src/AbstractSqlDriver.ts b/packages/knex/src/AbstractSqlDriver.ts
index f4c69231cb01..083f6eb3fda6 100644
--- a/packages/knex/src/AbstractSqlDriver.ts
+++ b/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);
diff --git a/tests/EntityHelper.mysql.test.ts b/tests/EntityHelper.mysql.test.ts
index dadb31a884b2..54fbdf601f6a 100644
--- a/tests/EntityHelper.mysql.test.ts
+++ b/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);