From 99556620a95da8fbfc7bca71e1a904f5f54f08be Mon Sep 17 00:00:00 2001
From: Brian Pandola <bpandola@hsdp.io>
Date: Wed, 14 Oct 2020 08:32:42 -0700
Subject: [PATCH] Fix: Empty sets not removed from item after UpdateExpression
 (#3386)

DynamoDB does not support empty sets. If the last item in a set is
deleted as part of an UpdateExpression, the attribute must be removed.

Ref: https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/HowItWorks.NamingRulesDataTypes.html

Fixes #3296
---
 moto/dynamodb2/parsing/executors.py   |  7 ++++++
 tests/test_dynamodb2/test_dynamodb.py | 34 +++++++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/moto/dynamodb2/parsing/executors.py b/moto/dynamodb2/parsing/executors.py
index 2f2f2bb8..76642542 100644
--- a/moto/dynamodb2/parsing/executors.py
+++ b/moto/dynamodb2/parsing/executors.py
@@ -161,6 +161,13 @@ class DeleteExecutor(NodeExecutor):
                 # DynamoDB does not mind if value is not present
                 pass
 
+        # DynamoDB does not support empty sets.  If we've deleted
+        # the last item in the set, we have to remove the attribute.
+        if not string_set_list:
+            element = self.get_element_to_action()
+            container = self.get_item_before_end_of_path(item)
+            container.pop(element.get_attribute_name())
+
 
 class RemoveExecutor(NodeExecutor):
     def execute(self, item):
diff --git a/tests/test_dynamodb2/test_dynamodb.py b/tests/test_dynamodb2/test_dynamodb.py
index e2dd744e..06dfec01 100644
--- a/tests/test_dynamodb2/test_dynamodb.py
+++ b/tests/test_dynamodb2/test_dynamodb.py
@@ -5445,3 +5445,37 @@ def test_lsi_projection_type_keys_only():
     items[0].should.equal(
         {"partitionKey": "pk-1", "sortKey": "sk-1", "lsiK1SortKey": "lsi-sk"}
     )
+
+
+@mock_dynamodb2
+def test_set_attribute_is_dropped_if_empty_after_update_expression():
+    table_name, item_key, set_item = "test-table", "test-id", "test-data"
+    client = boto3.client("dynamodb", region_name="us-east-1")
+    client.create_table(
+        TableName=table_name,
+        KeySchema=[{"AttributeName": "customer", "KeyType": "HASH"}],
+        AttributeDefinitions=[{"AttributeName": "customer", "AttributeType": "S"}],
+        ProvisionedThroughput={"ReadCapacityUnits": 5, "WriteCapacityUnits": 5},
+    )
+
+    client.update_item(
+        TableName=table_name,
+        Key={"customer": {"S": item_key}},
+        UpdateExpression="ADD orders :order",
+        ExpressionAttributeValues={":order": {"SS": [set_item]}},
+    )
+    resp = client.scan(TableName=table_name, ProjectionExpression="customer, orders")
+    item = resp["Items"][0]
+    item.should.have.key("customer")
+    item.should.have.key("orders")
+
+    client.update_item(
+        TableName=table_name,
+        Key={"customer": {"S": item_key}},
+        UpdateExpression="DELETE orders :order",
+        ExpressionAttributeValues={":order": {"SS": [set_item]}},
+    )
+    resp = client.scan(TableName=table_name, ProjectionExpression="customer, orders")
+    item = resp["Items"][0]
+    item.should.have.key("customer")
+    item.should_not.have.key("orders")