How to append a value to list attribute on AWS DynamoDB?
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'])
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.
I would look at update expressions: http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.Modifying.html#Expressions.Modifying.UpdateExpressions.ADD
Should be doable with an ADD, although not sure what the support in boto is for this.