mock_dynamodb2 fails to update field that is used in a Global Secondary Index

In the newest version of moto 2.0.1, when calling update_item on a field that is used as a key in a GSI, the operation triggers a ValidationException saying that an AttributeValue may not contain an empty string.

how to reproduce the issue

Save the following code to .py file and run.

from moto import mock_dynamodb2
import boto3

with mock_dynamodb2():
    name = "TestTable"
    conn = boto3.client("dynamodb", region_name="eu-west-2", aws_access_key_id="ak", aws_secret_access_key="sk")
    conn.create_table(
        TableName=name,
        KeySchema=[{"AttributeName": "main_key", "KeyType": "HASH"}],
        AttributeDefinitions=[{"AttributeName": "main_key", "AttributeType": "S"},
            {"AttributeName": "index_key", "AttributeType": "S"}],
        ProvisionedThroughput={"ReadCapacityUnits": 5, "WriteCapacityUnits": 5},
        GlobalSecondaryIndexes=[
            {
                "IndexName": "test_index",
                "KeySchema": [
                    {
                        "AttributeName": "index_key",
                        "KeyType": "HASH"
                    }
                ],
                "Projection": {
                    "ProjectionType": "ALL",
                },
                "ProvisionedThroughput": {"ReadCapacityUnits": 1, "WriteCapacityUnits": 1}
            }
        ]
    )

    conn.put_item(
        TableName=name,
        Item={
            "main_key": {"S": "testkey1"},
            "extra_data": {"S": "testdata"},
            "index_key": {"S": "indexkey1"}
        },
    )

    resp = conn.update_item(
        TableName=name,
        Key={"main_key": {"S": "testkey1"}},
        UpdateExpression="set index_key=:new_index_key",
        ExpressionAttributeValues={":new_index_key": {"S": "new_value"}}
    )

    print(resp)

what you expected to happen That the attribute index_key updates successfully.

I have validated that this code runs correctly against dynamodb proper without issues. It also works under the 1.3.6 version of moto.

what actually happens

Traceback (most recent call last):
  File "test.py", line 41, in <module>
    resp = conn.update_item(
  File "/home/h0wser/.local/lib/python3.8/site-packages/botocore/client.py", line 357, in _api_call
    return self._make_api_call(operation_name, kwargs)
  File "/home/h0wser/.local/lib/python3.8/site-packages/botocore/client.py", line 676, in _make_api_call
    raise error_class(parsed_response, operation_name)
botocore.exceptions.ClientError: An error occurred (ValidationException) when calling the UpdateItem operation: One or more parameter values were invalid: An AttributeValue may not contain an empty string

what version of Moto you’re using

boto==2.49.0
boto3==1.17.22
botocore==1.20.22
moto==2.0.1

Installed via pip.

[bblommers]

This is now part of moto >= 2.0.10 @JamesBelchamber

[bblommers]

That will depend on when another maintainer approves the fix @JamesBelchamber - I have no control over that timeline I’m afraid