How to update an item in a list attribute?

[barticus]

Hi,
I have a model with a list attribute. I'm trying to update the element at a specific index.
I understand the dynamo way to do this is to do something like this:

aws dynamodb update-item \
    --table-name ProductCatalog \
    --key '{"Id":{"N":"789"}}' \
    --update-expression "SET RelatedItems[1] = :ri" \
    --expression-attribute-values file://values.json \
    --return-values ALL_NEW

(from docs)

But I'm struggling to work out the set syntax for that. I tried the below, but get the error OneTable execute failed "update" for "ScheduledEnvironment. The provided key element does not match the schema

(code adapted to match the above)

ProductCatalog.update(
    {
      Id: 789,
    },
    {
      set: {
        [`RelatedItems[${index}]`]: "@{item}",
      },
      substitutions: {
        item,
      },
    }
  );

[barticus]

Disregard, I realised the error was with the SK not resolving correctly.
The format I originally posted works, for anyone else looking into this.

[dev-embedthis]

Thanks for following up and clarifying.