Making Dynamodb update_item idempotent - List Attribute

Question

I would like to make my update_item operation idempotent. I have an attribute of type list, and I would like to add a element to list only if not exists. I imagine that i need to use: ConditionExpression

uptd = 'SET status_pedido_disponiveis = list_append(if_not_exists(status_pedido_disponiveis, :empty_list), :my_value)'
attr={ ":my_value": {"L": [{"S": xml }]}, ":empty_list":{"L": [] } }
self.dynamodb.update_item(TableName=self.table_name, Key={'order_id':{'S': order_id}}, 
                          UpdateExpression=uptd, 
                          ExpressionAttributeValues=attr
                          )

p.magalhaes · Accepted Answer · 2016-08-09 13:23:37Z

1

Here is the solution:

uptd = 'SET status_pedido_disponiveis = list_append(if_not_exists(status_pedido_disponiveis, :empty_list), :my_value)'
attr={ ":my_value": {"L": [{"S": xml }]}, ":empty_list":{"L": [] }, ':xml_content': {"S": xml } }
self.dynamodb.update_item(TableName=self.table_name, Key={'order_id':{'S': order_id}}, 
                          UpdateExpression=uptd, 
                          ExpressionAttributeValues=attr,
                          ConditionExpression = "not contains (status_pedido_disponiveis, :xml_content)"
                          )

answered Aug 9, 2016 at 13:23

p.magalhaes

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