Update an item in Dynamodb?

Question

I am a newbie to this technology . So bear with my below explanation.

My usecase :

I am updating an item in Dynamodb based on primary key(ID) . Basically my record has 4 columns ( ID, description, name , price ) .

The problem here is suppose when i have an item with key 123

ID description name price

123 Model      nokia  10

when i use above lamda function to update all the columns it is working fine. But when i try to update only the description column it is throwing error that 'ExpressionAttributeValues can't be empty` . So for that i had did some thing like this

ExpressionAttributeValues:{
     ":d":event.description ? event.description : null ,
     ":n":event.name ? event.name : null,
     ":p":event.price ? event.price : null
     }

But the problem with this is it will update the only value that i had given . Like if i had given only description to modal1 then it will change that and give the result like this

ID description name price

123 Model1  null  null

Which is right because that above snippet says to do that.

What i want to achieve ?

How can i change that behaviour to acheive like this

ID description name price

123 Model1     nokia  10

1. Is there a way that it will update only the passed parameters and keep the others to its previous values ?

Any help is appreciated

Updatedcode

var AWS = require("aws-sdk");
AWS.config.update({
    region: "regionname",
    endpoint: "endpointurl"
});
var docClient = new AWS.DynamoDB.DocumentClient()

var tablename = "Example"; 


var UpdateExpression = "'set";
var ExpressionAttributeValues= {};
var ExpressionAttributeNames= {};
var ID = "xx";
var description = "Iphone";
var name = "xx";
var price = 50;
 var params = { 
    TableName: tablename, 
    Key:{ 
    "ID": ID 
    }
    };
docClient.update(params,onUpdate);
 function onUpdate(err) {
    if (err) {
        console.error("Unable to scan the table. Error JSON:", JSON.stringify(err, null, 2));
    } else {
        console.log("Update succeeded.");
    }
    if(description) { 
        UpdateExpression = UpdateExpression + " description = :d,",
        ExpressionAttributeValues [":d"] =  description ,
        console.log("first...")

        }
        if(name){ 
        UpdateExpression = UpdateExpression + " #na = :n,",
        ExpressionAttributeNames['#na'] = 'name' ,
        ExpressionAttributeValues[":n"] = name ,
        console.log("second...")
        }
        if(price){ 
        UpdateExpression = UpdateExpression + " price = :p',",
        ExpressionAttributeValues[":p"] = price ,
        console.log("third...")
        }        
        console.log(UpdateExpression)
        console.log(ExpressionAttributeNames)
        console.log(ExpressionAttributeValues)
        return (UpdateExpression,ExpressionAttributeNames,ExpressionAttributeValues)
    }

When i ran the code i got output like this :

D:\\Dynamo>node FullUpdate.js

Update succeeded.
first...
second...
third...
'set description = :d, #na = :n, price = :p',
{ '#na': 'name' }
{ ':d': 'Iphone', ':n': 'xx', ':p': 50 }

Answer 1

You have to fabric your query depending on the variables you have, so you have to do something like this

UpdateExpression = "set";
ExpressionAttributeValues: {};
ExpressionAttributeNames: {};

if(event.description){
 UpdateExpression = UpdateExpression + "description = :d";
 ExpressionAttributeValues[":d"] = event.description
}
if(event.name){
 UpdateExpression = UpdateExpression + "#na = :n";
 ExpressionAttributeNames['#na'] = 'name'
 ExpressionAttributeValues[":d"] = event.name
}
if(event.price){
 UpdateExpression = UpdateExpression + "price = :p";
 ExpressionAttributeValues[":d"] = event.pice
}

be carful of the comma, and log your query to be sure it fits your expectations. you can check a function i use for this purpose,

update(_id, store_id, opts?) {

    var Key = {
      _id: _id,
      store_id: store_id
    };

    if (Object.keys(opts).length > 0) {
      let UpdateExpression =
        "SET #date = :date, #author = :author";

      let ExpressionAttributeNames = {
        "#date": "date",
        "#author": "author"
      }

      let ExpressionAttributeValues = {
        ":date": new Date().getTime(),
        ":author": localStorage.getItem('user_id')
      }

      if ((opts && opts.title)) {
        UpdateExpression = UpdateExpression + "#title = :title,";
        ExpressionAttributeNames["#title"] = "title";
        ExpressionAttributeValues[":title"] = opts.title
      }

      if ((opts && opts.category)) {
        UpdateExpression = UpdateExpression + "#category = :category,";
        ExpressionAttributeNames["#category"] = "category";
        ExpressionAttributeValues[":category"] = opts.category
      }

      if ((opts && opts.category)) {
        UpdateExpression = UpdateExpression + "#dimension = :dimension,";
        ExpressionAttributeNames["#dimension"] = "dimension";
        ExpressionAttributeValues[":dimension"] = opts.dimension
      }

      if ((opts && opts.category)) {
        UpdateExpression = UpdateExpression + "#destination = :destination,";
        ExpressionAttributeNames["#destination"] = "destination";
        ExpressionAttributeValues[":destination"] = opts.destination
      }

      if ((opts && opts.category)) {
        UpdateExpression = UpdateExpression + "#timeStart = :timeStart,";
        ExpressionAttributeNames["#timeStart"] = "timeStart";
        ExpressionAttributeValues[":timeStart"] = opts.timeStart
      }

      if ((opts && opts.category)) {
        UpdateExpression = UpdateExpression + "#timeEnd = :timeEnd,";
        ExpressionAttributeNames["#timeEnd"] = "timeEnd";
        ExpressionAttributeValues[":timeEnd"] = opts.timeEnd
      }

      if ((opts && opts.publish)) {
        UpdateExpression = UpdateExpression + "#publish = :publish,"
        ExpressionAttributeNames["#publish"] = "publish"
        ExpressionAttributeValues[":publish"] = "x";
      }


      UpdateExpression = UpdateExpression.replace(new RegExp(',$'), '');

      if ((opts && opts.nbviews)) {
        UpdateExpression = UpdateExpression + "ADD #nbviews = :nbviews"
        ExpressionAttributeNames["#nbviews"] = "nbviews"
        ExpressionAttributeValues[":nbviews"] = 1;
      }

      if ((opts && !opts.publish)) {
        UpdateExpression = UpdateExpression + " REMOVE #publish"
        ExpressionAttributeNames["#publish"] = "publish"
      }

      return this.dataService.update(TABLENAME, Key, UpdateExpression, null, ExpressionAttributeNames, ExpressionAttributeValues);