DynamoDB Update JSON - amazon-web-services

Question

I have a dynamodb table that backs a shopping cart. The schema is CartKey then a List of Maps that contain a CartItemId. Is there a way to update a cart item, which is nested in the list of maps, based on the CartKey and a CartItemId.
Thanks

Answer 1

I'm in search for a solution to the same issue. Unfortunately I don't think one is available.
In mature document-based DBs (such as MongoDB) you should be able to specify a queried index (see https://docs.mongodb.org/manual/reference/operator/update/positional/#up.S), but DynamoDB doesn't support that.
The next best thing is to query the Cart document with the entire CartItems array, iterate it to find the index of your CartItem and do a conditional write. (For example: update the document and set CartItems[7].Quantity to 4 only if CartItems[7].ProductId is "WSK-1234")
Yes you need to do a read before a write and perform some client-side searching, but at least you can be sure you aren't updating the wrong item.

Answer 2

I would change your data model from a list of maps, to a map of maps where the keys are CartItemId's.
Example document:
{
CartKey : 'Cart-123',
items : : {
CartItemId1 : { quantity : 1, productId: "pid-123" },
CartItemId2 : { quantity : 4, productId: "pid-987" }
}
}
Then you can perform update expressions to specific CartItems.
UpdateExpression: "set items.CartItemId1.quantity = 2"

Answer 3

I did something similar with a map of maps and it worked for me. Hopefully this will be helpful.
$RegID = "abracadabra";
$tableName="DefaultDelivery";
$marshaler = new Marshaler();
$requested_delivery = '{"Packet0":{"PacketNo":"2","Quantity":"1000ml","Type":"Toned Milk"},"Packet2":{"PacketNo":"4","Quantity":"250ml","Type":"Toned Milk"}}';
$eav = $marshaler->marshalJson('
{
":RequestedDelivery" : '.$requested_delivery.'
}
');
$key = $marshaler->marshalJson('
{
"RegistrationID" : "'.$RegID.'"
}
');
$params = [
'TableName' => "$tableName",
'Key' => $key,
'ExpressionAttributeValues' => $eav,
'UpdateExpression' => 'SET RequestedDelivery = :RequestedDelivery',
'ReturnValues' => 'UPDATED_NEW'
];
try {
$result = $client->updateItem($params);
echo "SUCCESS";
}
catch (DynamoDbException $e){
echo "Unable to update Item : \n";
}