DynamoDB - DELETE item from nested sets

moto/dynamodb2/models.py

@@ -420,12 +420,18 @@ class Item(BaseModel):
 
                     if not dyn_value.is_set():
                         raise TypeError
-                    existing = self.attrs.get(key, None)
+                    key_head = key.split(".")[0]
+                    key_tail = ".".join(key.split(".")[1:])
+                    existing = self.attrs.get(key_head)
+                    existing = existing.get(key_tail)
                     if existing:
                         if not existing.same_type(dyn_value):
                             raise TypeError
                         new_set = set(existing.value).difference(dyn_value.value)
-                        self.attrs[key] = DynamoType({existing.type: list(new_set)})
+                        existing.set(
+                            key=None,
+                            new_value=DynamoType({existing.type: list(new_set)}),
+                        )
                 else:
                     raise NotImplementedError(
                         "{} update action not yet supported".format(action)

tests/test_dynamodb2/test_dynamodb_table_with_range_key.py

@@ -1347,7 +1347,7 @@ def test_update_item_add_with_expression():
 
 
 @mock_dynamodb2
-def test_update_item_add_with_nested_sets_and_expression_names():
+def test_update_item_add_with_nested_sets():
     table = _create_table_with_range_key()
 
     item_key = {"forum_name": "the-key", "subject": "123"}
@@ -1383,6 +1383,32 @@ def test_update_item_add_with_nested_sets_and_expression_names():
     dict(table.get_item(Key=item_key)["Item"]).should.equal(current_item)
 
 
+@mock_dynamodb2
+def test_update_item_delete_with_nested_sets():
+    table = _create_table_with_range_key()
+
+    item_key = {"forum_name": "the-key", "subject": "123"}
+    current_item = {
+        "forum_name": "the-key",
+        "subject": "123",
+        "nested": {"str_set": {"item1", "item2", "item3"}},
+    }
+
+    # Put an entry in the DB to play with
+    table.put_item(Item=current_item)
+
+    # Update item to add a string value to a nested string set
+    table.update_item(
+        Key=item_key,
+        UpdateExpression="DELETE nested.str_set :v",
+        ExpressionAttributeValues={":v": {"item3"}},
+    )
+    current_item["nested"]["str_set"] = current_item["nested"]["str_set"].difference(
+        {"item3"}
+    )
+    dict(table.get_item(Key=item_key)["Item"]).should.equal(current_item)
+
+
 @mock_dynamodb2
 def test_update_item_delete_with_expression():
     table = _create_table_with_range_key()