Laravel, appending an attribute to a returned json appends the entire object - laravel

Question

I have 2 tables Customer(Parent) and jobCode(Child) with one to many relationships
I want to retrieve a child record but with one extra field from the parent (customerGuid) as well. My code:
$jobCodes=JobCode::all();
foreach ($jobCodes as $jobCode) {
$jobCode['customerGuid']=$jobCode->customer->guid;
// I also tried $jobCode->setAttribute('customerGuid',$jobCode->customer->guid);
}
return $jobCodes;
I was expecting just the parent's guid field to be appended to each jobCode object and returned. However the ENTIRE customer parent object is returned!
[{
"id": 137,
"customerId": 1,
"jobCode": "Journeyman Plumber",
"jobDescription": "Journeyman Plumber",
"created_at": null,
"updated_at": "2017-01-27 12:20:27",
"guid": "28f35e94-e483-11e6-98e9-e0db55883624",
"customerGuid": "8d48931d-dc61-11e6-8927-e0db55883624",
"customer": {
"id": 1,
"name": "ACME",
"address1": "",
"address2": "",
"city": "San Jose",
"zip": "",
"phone": "",
"fax": "",
"email": "",
"guid": "8d48931d-dc61-11e6-8927-e0db55883624",
"stateName": "California",
"created_at": null,
"updated_at": "2017-01-20 07:10:59"
}
}, {
"id": 138,
"customerId": 1,
"jobCode": "JRP PreFab",
"jobDescription": "JRP",
......,

Answer 1

Its because your return statement returns $jobCodes; which is the entire table you retrieve here: $jobCodes=JobCode::all();
try putting this inside your loop
echo '$jobCode' to see what is the output every time it loops

Answer 2

Reaching for the relationship customer on $jobCode you automatically lazy load the relationship object, which is stored in $jobCode->customer attribute.
Try this instead just returning the collection:
// return $jobCodes;
return $jobCodes->map(function($jobCode){
$jobCode = $jobCode->toArray();
unset($jobCode['customer']);
return $jobCode;
});