I'm trying to read a set of records from a custom Big Object, update a long text field (not part of the object index), and then upsert back. When trying to update the value of the field I get 'Field is not writable'. All fields have update permissions for my running user and I'm able to insert brand new records fine, its just trying to update the field that throws this error. Is there an undocumented limitation or am I doing something fundamentally wrong?
List<myObject__b> mo= [SELECT longtextfield__c FROM myObject__b LIMIT 1];
for(myObject__b o : mo){
o.longtextfield__c = 'new value'; //throws a not writable error
}
database.insertImmediate(mo);
longtextfield__c
is not Read only?