Update only the given column value with appsync api and datasource as posgresql - postgresql

Question

I have created appsync api with postgresql as datasource. Now it's very straight to create , update and delete any record with graphql. But i want something like, whenever any update takes places , i can update value of that particular column only. Eg
lets we have user table with 3 fields:
user_id
name
city
now when we update city for user than only city should update:
mutation update{
updateUser(input:{user_id:"xxxxx",city:"xyz"}){
user_id
name
city
}}
user update input:
input updateUserInput{
user_id!
name
city
}
Is there any way to do this in vtx template or we have to use lambda as datasource and then do this.

Answer 1

I figured it out how we can achieve it:
you can do like this:
#set ($updates = "")
#foreach( $item in $ctx.args.input.keySet())
#if($item)
#set ($updates = "$updates, $item = '$ctx.args.input.get($item)'")
#end
#end
--------used cognito as authentication , so user id will come in indentity
#set($user_id = ($ctx.identity.username))
#set($updates = "$updates, updated_at = '$util.time.nowFormatted('yyyy-MM-dd HH:mm:ssZ')'")
{
"version": "2018-05-29",
"statements": [
$util.toJson("update public.tableName SET $updates.substring(1) WHERE user_id = '$user_id'"),
$util.toJson("select * from public.tableName WHERE user_id = '$user_id'")
]
}
I hope it will help someone.