CODEWITHSUNDEEP
python-3.xamazon-web-servicesamazon-dynamodb
Bharath Bharath
Bharath Bharath

Reputation: 99

How to add an attribute to the dynamoDB table item if it does not exist and add value to that attribute?

I'm trying to add value to the attribute "dataname" which is type LIST in the dynamoDB table.

        response = table.update_item(
            Key={"name": xyz},
            UpdateExpression="SET dataname = list_append(dataname, :i)"
            ExpressionAttributeValues={
                ':i': [1200, readonly],
            },
        )

The above logic works only if the attribute "dataname" is already present in the item. If the attribute is not present it gives the following error: "The provided expression refers to an attribute that does not exist in the item".

Is there a way to add the attribute "dataname" if it does not exist in the item and then update the values ? I'm using Python language for this.

Upvotes: 3

Views: 3914

Answers (1)

Maurice
Maurice

Reputation: 13197

I recently used an implementation like this:

import boto3

update_expression = "SET #dataname = list_append(if_not_exists(#dataname, :empty_list), :i)"

dynamodb = boto3.resource("dynamodb")
result_table = dynamodb.Table(TABLE_NAME)

result_table.update_item(
    Key={
        "name": "xyz",
    },
    UpdateExpression=update_expression,
    ExpressionAttributeNames={
        "#dataname": "dataname",
    },
    ExpressionAttributeValues={
        ":empty_list": [],
        ":i": [1200, readonly],
)

This is adapted from an implementation for NodeJS I found on stackoverflow by Nasreen Ustad) and translated to Python. (Looks like there is a sort of canonical answer for this without the complete implementation)

It basically works like this:

  1. It checks if the #dataname attribute exists, if that's the case it is used, otherwise the value of :empty_list will be used in its place
  2. Then the new value(s) will be appended to the list.

Upvotes: 1

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