example UpdateItem with conditions

updateItem.vtl

{
  "version" : "2017-02-28",
  "operation" : "UpdateItem",
  "key" : {
    "createdAt": $util.dynamodb.toDynamoDBJson($ctx.args.createdAt),
    "translationState": $util.dynamodb.toDynamoDBJson('waiting')
  },
   "update" : {
    "expression" : "SET #content = :content",
    "expressionNames" : {
        "#content" : "content"
    },
    "expressionValues" : {
        ":content" : $util.dynamodb.toDynamoDBJson("hello")
    }
  },
  "condition" : {
    "expression" : "#translationState = :translationState AND #translationKey = :translationKey",
    "expressionNames" : {
        "#translationState" : "translationState",
        "#translationKey" : "translationKey"
    },
    "expressionValues" : {
        ":translationState": $util.dynamodb.toDynamoDBJson('waiting'),
        ":translationKey": $util.dynamodb.toDynamoDBJson($ctx.args.id)
    },
    }
}