Invalid identifier: Amount__c in trigger in Salesforce - salesforce

Question

I have a trigger on a master-detail object where Doctor is master and Patient is child. Doctor has a field called TotalAmount and Patient has a field called Amount. Now, when a patient is filling in the amount field, the TotalAmount field in Doctor should give the sum of Amount in all the patient records.
I have written the code below, but it shows an error:
Invalid identifier: Amount__c
How can I fix this?
trigger tgPatient on Patient__c (after insert,after update) {
Set<Id>SetDoctor = new Set<Id>();
for (Patient__c p : trigger.new) {
if( p.Amount__c != Null ) {
SetDoctor.add(p.Doctor__c);
}
}
List<Doctor> lstDoctor = new List<Doctor>();
for( Doctor__c d : [SELECT Id, (SELECT Id,Amount__c FROM Patients__r) FROM Doctor__c WHERE Id IN:SetDoctor]){
Integer Amount__c = 0;
for (Patient__c p : d.Patient__r) {
int Amount__c += p.Amount__c;
}
d.Total_Amount__c = int Amount__c;
lstDoctor.add(d);
}
update lstDoctor;
}

Answer 1

Try the below one:
trigger tgPatient on Patient__c (after insert,after update) {
Set<Id>SetDoctor = new Set<Id>();
for (Patient__c p : trigger.new){
if( p.Amount__c != Null ){
SetDoctor.add(p.Doctor__c);
}
}
List<Doctor> lstDoctor = new List<Doctor>();
for( Doctor__c d : [SELECT Id, (SELECT Id,Amount__c FROM Patients__r) FROM Doctor__c WHERE Id IN:SetDoctor]){
Integer amountVAR = 0;
for (Patient__c p : d.Patient__r){
amountVAR += p.Amount__c;
}
d.Total_Amount__c = amountVAR ;
lstDoctor.add(d);
}
update lstDoctor;
regards
ajay

Answer 2

Try below:
trigger tgPatient on Patient__c(after insert, after update) {
Set < Id > SetDoctor = new Set < Id > ();
for (Patient__c p: trigger.new) {
if (p.Amount__c != Null) {
SetDoctor.add(p.Doctor__c);
}
}
List < Doctor > lstDoctor = new List < Doctor > ();
for (Doctor__c d: [SELECT Id, (SELECT Id, Amount__c FROM Patients__r) FROM Doctor__c WHERE Id IN: SetDoctor]) {
d.Total_Amount__c = 0;
for (Patient__c p: d.Patient__r) {
d.Total_Amount__c += p.Amount__c;
}
lstDoctor.add(d);
}
update lstDoctor;
}
But why do you want to use a Trigger? Why not create a roll-up summary field on Doctor?
Creating Roll Up Summary fields