defaultArg/if_not_exists for the same attribute

[matti-avilabs]

I'm trying to figure out if there's any way to achieve the same end result as is given in the Preventing overwrites of an existing attrubute section from here:

aws dynamodb update-item \
    --table-name ProductCatalog \
    --key '{"Id":{"N":"789"}}' \
    --update-expression "SET Price = if_not_exists(Price, :p)" \
    --expression-attribute-values '{":p": {"N": "100"}}' \
    --return-values ALL_NEW

So far I haven't been successful in any of my attempts so wondering if there's something I'm missing? From digging around it seems if defaultArg would be the way to go but as far as I can tell the only way to construct a valid update expression using that is if the attribute being set and the one passed into defaultArg are not the same ones.

I've tried the following but the resulting expressions are invalid in both cases:

For optional attributes: SET p.Price (defaultArg p.Price price |> Some) For non-optional attributes: SET p.Price (defaultArg (Some p.Price) price)