dreamfactory php脚本pre_process有效负载修改一个资源字段

Question

In Dreamfactory rest API within a post request I receive a payload like this :

{"resource": [{"Time":"2018-12-21T07:49:23","Gas":"111111","Pressure":"1111","Temperature":"11111"}]}

how can I refer only to field "Gas" to modify it into "00000" before insert it into mysql database ? This is the script that I write but I recive the below error when I do a post request:

<?php

$event['request']['payload']['resource']['Gas']="00000";
$event['request']['content']=json_encode($event['request']['payload']);

?>

the error is:

{"error":{"code":1000,"context":{"error":["Gas"],"resource":{"0":{"messageID":32},"Gas":{"code":500,"message":"array_merge(): Argument #1 is not an array"}}},"message":"Batch Error: Not all requested records could be created.","status_code":500}}