DynamoDB: Update atomic counter and add other info if new item

Question

I have a db with only one hash key, and two numeric values: counter and some_value .

I want to update the atomic counter counter . I know that boto /DyamoDb will take care for me of the creation of the element if not already present in the db.

Since I am creating the element, I'd like to use the same write operation to also put some_value in the db. Is it possible at all to modify the update_item code in that respect?

table.update_item(
            Key={
                'my_key': some_string
            },
            UpdateExpression="SET counter = if_not_exists(counter , :start) + :inc",
            ExpressionAttributeValues={
                ':inc': 1,
                ':start' : 0
            },
            ReturnValues="UPDATED_NEW"
        )

Answer 1

Yes , you can also update some_item along with your counter. In fact, you can update multiple items. Below is the tried and tested code for updating multiple values.

    table.update_item(
    Key={
        'my_key': some_string
    },
    UpdateExpression="SET #counter = if_not_exists(#counter, :start) + :inc, #some_value = if_not_exists(#some_value, :start) - :inc",
    ExpressionAttributeNames={'#counter': 'counter', '#some_value': 'some_value'},
    ExpressionAttributeValues={
        ':inc': 1,
        ':start': 0
    },
    ReturnValues="UPDATED_NEW"
)

PS: Always define your expression attribute names rather than directly using it in the expression to avoid conflicts and subsequent errors.