node.js passing a parameter to DynamoDB updateItem method

Question

I want to write a function that updates given parameter in dynamodb.

For example in a dynamodb table where each userId is the key I have values like

{ "categoryname": "a", "skillState": "a", "skipcount": 1, "userId": "amzn1.ask.account.xxx” }

I wanna set the "categoryname": "b" although there might be 10-15 fields like this so I dont wanna hard code the field name.

function (userId,itemToUpdate,itemValue,callback) {


    var updateExpressionString = "SET #"+itemToUpdate+" =:val1";
    var expressionAtt = '#'+itemToUpdate + '';

    console.log(updateExpressionString)
    console.log(expressionAtt)

    this.dynamodb.updateItem({
            TableName: constants.dynamoDBDetailTableName,
            Key: {
                userId: {
                    S: userId
                }
            },
            UpdateExpression: updateExpressionString,
            ExpressionAttributeNames : {
                   expressionAtt : itemToUpdate
            },
            ExpressionAttributeValues : {
                ':val1': {'S':itemValue}
            }
        }, function (err, data) {
            if (err) {
                console.log(err)
                console.log('Error ')
            } else if (data.Item === undefined) {

            }else {
                  console.log(data)  
            }
    });
}

In ExpressionAttributeNames:

{ ValidationException: ExpressionAttributeNames contains invalid key: Syntax error; key: "expressionAtt"

This throws error obviously thinking that expressionAtt is the key while it is a local variable.

I am new to node.js , how can pass the local variable in to ExpressionAttributeNames and ExpressionAttributeValues

Answer 1

One way of dealing with this could be to pull the object out of updateItem, put it into its own variable like so:

var item = {
    TableName: constants.dynamoDBDetailTableName,
    Key: {
        userId: {
            S: userId
        }
    },
    UpdateExpression: updateExpressionString,
    ExpressionAttributeNames: {},
    ExpressionAttributeValue: {
        ':val1': {'S': itemValue}
    }
};

item.ExpressionAttributeNames[expressionAtt] = itemToUpdate;

this.dynamodb.updateItem(item);

I believe that will fix your problem