using dynamodb SET to append to a string set

Question

I have looked at recent posts and nothing on them has worked for me. I have a string set called "friendRequests" in my DynamoDB table and I am trying to append an element to it. However, I keep getting many errors when I try to call db.updateItem with these parameters. Here is my current code, the error is with ExpressionAttributeValues but I have probably spent an hour changing my syntax to no avail.

    var params = {
    TableName: "users",
    Key: { "username": { "S": addFriendInfo.friendUsername } },
    UpdateExpression: "SET friendRequests = list_append(friendRequests, :newFriend)",
    ExpressionAttributeValues: {
        ':newFriend': { "SS" : [addFriendInfo.username] }
    }
}

That is my code above. addFriendInfo.username / friendUsername are both just strings. This currently gives me a 'Invalid UpdateExpression: Incorrect operand type for operator or function; operator or function: list_append, operand type: SS'. I have tried many things. Can anyone point me in right direction to fixing this damn syntax?

Nadav Har'El Nadav Har'El · Accepted Answer · 2020-12-07T08:00:30

As the DynamoDB documentation explains, DynamoDB has two similar but distinct types - a list and a set. A list is an ordered list of items of any type, while a set is a non-empty, non-ordered collection of items of the same type. The "SS" type you used is a "set of strings". This is distinct from a "list", and you cannot use list_append on sets, as the error message tells you.

To add an element to a set (or create a new set if it doesn't yet exist) you can use the ADD operation, e.g., ADD friendRequests :newFriend.

using dynamodb SET to append to a string set

1 Answers