dynamodb atomic increment with condition-CodePudding

I would like to increment a counter in a row only if a certain attribute doesnt exist in the row. I tried this

response = dynamodb.update_item(TableName=table_name, 
        Key={'id':{'S': id}}, 
        UpdateExpression='SET #att = #att   :inc',  
        ConditionExpression="attribute_not_exists(#a)",
        ExpressionAttributeNames={"#att": "counter", "#a": "attribute"}, 
        ExpressionAttributeValues={':inc': {'N': '1'}}, 
        ReturnValues="UPDATED_NEW")

but I get an error

An error occurred (ValidationException) when calling the 
UpdateItem operation: The provided expression refers to an 
attribute that does not exist in the item

CodePudding user response：

Seems like attribute exists on the item but counter doesn’t, so you can’t access the current value of counter to increment it.

CodePudding user response：

Using "ADD" expression instead of "SET" will create the counter attribute (if it does not exist) then add the value: doc

The query code may look like:

response = dynamodb.update_item(TableName=table_name, 
        Key={'id':{'S': id}}, 
        UpdateExpression='ADD #att :inc',  
        ConditionExpression="attribute_not_exists(#a)",
        ExpressionAttributeNames={"#att": "counter", "#a": "attribute"}, 
        ExpressionAttributeValues={':inc': {'N': '1'}}, 
        ReturnValues="UPDATED_NEW")