Add non existing element to array in DynamoDB

Question

Im trying to update items attribute that is a list of strings. Can I update (append) the attribute only if it not exist . Kind of list_append & if_not_exists.

var params = { ...

UpdateExpression: 

'SET friends = list_append(if_not_exists(friends, :empty_list), :new_friend)',

ExpressionAttributeValues:{
    ":new_friend": [{"S":"Bobo"}],
    ":empty_list" :[]
 } };

this is not working, is there a way? so if Bobo still not in my "friends" list it will append it

Answer 1

You can use the "not contains" and "list_append" for your requirement.

The below code inserts the new friend to list if the friend is NOT already present in the list.

If the friend is already present in the list, it would throw "conditional request failed".

Error message if condition fails:-

Unable to update item. Error JSON: {
  "message": "The conditional request failed",
  "code": "ConditionalCheckFailedException",
  "time": "2016-06-22T08:18:36.483Z",
  "requestId": "86805965-240b-43e0-8fdc-77fb9ae1b15c",
  "statusCode": 400,
  "retryable": false,
  "retryDelay": 0
}

The below code works fine. It has been tested successfully.

Code Sample:

var AWS = require("aws-sdk");

AWS.config.update({
    region : "us-west-2",
    endpoint : "http://localhost:8000"
});

var docClient = new AWS.DynamoDB.DocumentClient();

var table = "users";

var userid = 1;
var friendId = ["f4"];
var friendIdStr = "f4";


//Add the new DOCUMENT TYPE attribute to the table
var params = {
    TableName : table,
    Key: {
        "id" : userid 
    },
    "UpdateExpression": "set friends = list_append (friends, :friendId)",
    "ConditionExpression": "not contains (friends, :friendIdStr)",
    "ExpressionAttributeValues": { 
        ":friendId": friendId,
        ":friendIdStr" : friendIdStr
    },
    "ReturnValues" : "UPDATED_NEW"
};

console.log("Updated an item...");
docClient.update(params, function(err, data) {
    if (err) {
        console.error("Unable to update item. Error JSON:", JSON.stringify(err,
                null, 2));
    } else {
        console.log("Updated item:", JSON.stringify(data, null, 2));
    }
});