DynamoDB PutItem keeps overwriting previous entry

Question

I want to put an order from my lex bot into dynamoDB however the PutItem operation overwrites each time(If the customer name is already in the table). I know from the documentation that it will do this if the primary key is the same.

My goal is to have each order put into the database so they will be easily searchable in the future. I have attached some screenshots below. Any help is appreciated

https://i.stack.imgur.com/8EEXR.jpg

def putDynam(orderNum, table_custName, slotKey, slotVal):
    client = boto3.resource('dynamodb')
    table = client.Table('blah')
    
    input = {'Customer': table_custName, 'OrderNumber':orderNum[0], 'Bun Type': slotVal[5], 'CheeseDecision': slotVal[1], 'Cheese Type': slotVal[0], 'Pickles': slotVal[4], 'SauceDecision': slotVal[3], 'Sauce Type': slotVal[2]}
    
    action = table.put_item(Item=input)

what is the primary key? is it unique for each order? because it has to be — LLL, Aug 07 '20 at 09:47
@LLL So at the moment the Primary Key is customer name, I don't quite understand how the Primary key can be unique for each entry. — PythonSmython, Aug 07 '20 at 09:52

score 1 · Answer 1 · answered Aug 07 '20 at 09:59

The primary key is used for identifying each item in the table. There can only be 1 record with a specific primary key (primary keys are unique).

Customer name is not a good primary key, because it's not unique.

In this case you could have an order with some generated Id (orderNumber in your example?), that could be the primary key, and Customer (preferably CustomerId) as a property.

Or you could have a composite primary key made up of CustomerId and OrderId.

If you want to query orders by customer, you could use an index if it's not in the primary key.

I recommend you read up on how DynamoDB works first. You can start with this data modelling tutorial from AWS.

score 1 · Answer 2 · answered Aug 07 '20 at 10:01

So, basically, the customer name has to be unique, since it's your Primary Key. You can't have two rows with the same primary key. A way could be to have an incremental value that serves as id, and each insert would simply have i+1 as its id.

You can see this stack overflow question for more information: https://stackoverflow.com/a/12460690/11593346

score 1 · Answer 3 · answered Aug 07 '20 at 10:19

Per https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/dynamodb.html#DynamoDB.Client.put_item, you can:

"perform a conditional put operation (add a new item if one with the specified primary key doesn't exist)"

Note

To prevent a new item from replacing an existing item, use a conditional expression that contains the attribute_not_exists function with the name of the attribute being used as the partition key for the table. Since every record must contain that attribute, the attribute_not_exists function will only succeed if no matching item exists.

Also see DynamoDB: updateItem only if it already exists

If you really need to know whether the item exists or not so you can trigger your exception logic, then run a query first to see if the item already exists and don't even call put_item. You can also explore whether using a combination of ConditionExpression and one of the ReturnValues options (for put_item or update_item) may return enough data for you to know if an item existed.

DynamoDB PutItem keeps overwriting previous entry

3 Answers3