AppSync DynamoDB Resolvers with update conditions

[kostyrin]

There is a resolver's code

{
  "version": "2017-02-28",
  "operation": "UpdateItem",
  "key": {
    "PK": $util.dynamodb.toDynamoDBJson($ctx.args.input.PK),
    "SK": $util.dynamodb.toDynamoDBJson($ctx.args.input.SK),
  },
  
    ## Set up some space to keep track of things you're updating **
    #set( $expNames  = {} )
    #set( $expValues = {} )
    #set( $expSet = {} )
    #set( $expAdd = {} )

    ## Increment "ActualLimit" 
    $!{expAdd.put("ActualLimit", ":actuallimit")}
    $!{expValues.put(":actuallimit", { "N" : $context.arguments.input.ActualLimit } )}

    ## Continue building the update expression, adding attributes you're going to ADD **
    #if( !${expAdd.isEmpty()} )
        #set( $expression = "${expression} ADD" )
        #foreach( $entry in $expAdd.entrySet() )
            #set( $expression = "${expression} ${entry.key} ${entry.value}" )
            #if ( $foreach.hasNext )
                #set( $expression = "${expression}," )
            #end
        #end
    #end
    
    ## Finally, write the update expression into the document, along with any expressionNames and expressionValues **
    "update" : {
        "expression" : "${expression}"
        #if( !${expNames.isEmpty()} )
            ,"expressionNames" : $utils.toJson($expNames)
        #end
        #if( !${expValues.isEmpty()} )
            ,"expressionValues" : $utils.toJson($expValues)
        #end
    },

    "condition" : {
        "expression"       : "LimitDate = :expectedLimitDate",
        "expressionValues" : {
            ":expectedLimitDate" : $util.dynamodb.toDynamoDBJson($context.arguments.input.LimitDate)
        }
    }

}

and mutation:

mutation MyMutation {
  update(input: {PK: "pk1", SK: "sk1", ActualLimit:0, LimitDate: "YYYY-MM-DD"}) {
    ActualLimit CreatedAt LimitDate
  }
}

where PK - primary key, SK - sorted key, ActualLimit is integer value, LimitDate is string value.

My question is how to write the code if I need two cases:

• If there is a record with same LimitDate value then need to increment ActualLimit for value from the mutation and SET fields
• If there is no record with same LimitDate value then need to just SET fields(like LimitDate)

e.g.: have record in DynamoDb: { "PK": "pk1", "SK": "sk1", "ActualLimit": 10, "LimitDate": "2022-06-08"} case 1:

mutation MyMutation {
  update(input: {PK: "pk1", SK: "sk1", ActualLimit:2, LimitDate: "2022-06-08"}) {
    ActualLimit CreatedAt LimitDate
  }
}

then have record in DynamoDb: { "PK": "pk1", "SK": "sk1", "ActualLimit": 12, "LimitDate": "2022-06-08"}

case 2:

mutation MyMutation {
  update(input: {PK: "pk1", SK: "sk1", ActualLimit:1, LimitDate: "2022-06-19"}) {
    ActualLimit CreatedAt LimitDate
  }
}

then have record in DynamoDb: { "PK": "pk1", "SK": "sk1", "ActualLimit": 1, "LimitDate": "2022-06-19"}

How to do that? thanks.