How to append a value to list attribute on AWS DynamoDB?

Question

I'm using DynamoDB as an K-V db (cause there's not much data, I think that's fine) , and part of 'V' is list type (about 10 elements). There's some session to append a new value to it, and I cannot find a way to do this in 1 request. What I did is like this:

item = self.list_table.get_item(**{'k': 'some_key'}) item['v'].append('some_value') item.partial_save() 

I request the server first and save it after modified the value. That's not atomic and looks ugly. Is there any way to do this in one request?

Answer 1

The following code should work with boto3:

table = get_dynamodb_resource().Table("table_name") result = table.update_item(     Key={         'hash_key': hash_key,         'range_key': range_key     },     UpdateExpression="SET some_attr = list_append(some_attr, :i)",     ExpressionAttributeValues={         ':i': [some_value],     },     ReturnValues="UPDATED_NEW" ) if result['ResponseMetadata']['HTTPStatusCode'] == 200 and 'Attributes' in result:     return result['Attributes']['some_attr'] 

The get_dynamodb_resource method here is just:

def get_dynamodb_resource():     return boto3.resource(             'dynamodb',             region_name=os.environ['AWS_DYNAMO_REGION'],             endpoint_url=os.environ['AWS_DYNAMO_ENDPOINT'],             aws_secret_access_key=os.environ['AWS_SECRET_ACCESS_KEY'],             aws_access_key_id=os.environ['AWS_ACCESS_KEY_ID']) 

Answer 2

You can do this in 1 request by using the UpdateItem API in conjunction with an UpdateExpression. Since you want to append to a list, you would use the SET action with the list_append function:

SET supports the following functions:

...

• list_append (operand, operand) - evaluates to a list with a new element added to it. You can append the new element to the start or the end of the list by reversing the order of the operands.

You can see a couple examples of this on the Modifying Items and Attributes with Update Expressions documentation:

• The following example adds a new element to the FiveStar review list. The expression attribute name #pr is ProductReviews; the attribute value :r is a one-element list. If the list previously had two elements, [0] and [1], then the new element will be [2].

  SET #pr.FiveStar = list_append(#pr.FiveStar, :r) 

• The following example adds another element to the FiveStar review list, but this time the element will be appended to the start of the list at [0]. All of the other elements in the list will be shifted by one.

  SET #pr.FiveStar = list_append(:r, #pr.FiveStar) 

The #pr and :r are using placeholders for the attribute names and values. You can see more information on those on the Using Placeholders for Attribute Names and Values documentation.