Is it possible to combine if_not_exists and list_append in update_item

amazon-dynamodb boto3
fabian picture fabian · Jan 22, 2016 · Viewed 18.4k times · Source

I'm trying to use the update_item functionality for DynamoDB in boto3.

I'm struggling right now to update lists for items. I would like to create a new list if the list does not exist yet and otherwise append to the existing list.

Using an UpdateExpression of the form SET my_list = list_append(my_list, :my_value) returns an error "The provided expression refers to an attribute that does not exist in the item" if the list does not exist yet.

Any idea how I would have to modify my UpdateExpression?

Thanks & best regards, Fabian

Answer

Boris Serebrov picture Boris Serebrov · Jan 25, 2016

You can use list_append(if_not_exists()) construction.

UpdateExpression:

'SET my_list2 = list_append(if_not_exists(my_list2, :empty_list), :my_value)'

ExpressionAttributeValues:

{ ":my_value":[{"S":"test"}], ":empty_list":{"L":[]} }

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