Count of Subscriptions on Contract

Question

There are two object Contract and Subscription.There is Lookup relationship between both. Contract is parent and Subscription is child object.

The child-relationship name between two is: Subscriptions

I have written Trigger on Subscription object to Count number of subscriptions and display that to Contract. Below is my trigger:

trigger CountContract on SBQQ__Subscription__c (after INSERT, after UPDATE, 
after DELETE ) {
Set <Id> contractIds = new Set <Id>();
List <Contract> lstContractsToUpdate = new List <Contract>();
if(Trigger.isInsert){
for(SBQQ__Subscription__c s:trigger.new){
    contractIds.add(s.SBQQ__Contract__r.id);
}
}
if(Trigger.isUpdate|| Trigger.isDelete){
for(SBQQ__Subscription__c s:trigger.old){
    contractIds.add(s.SBQQ__Contract__r.id);
 }
}
system.debug(contractIds);
for(Contract con:[SELECT Id,Number_of_Subscriptions__c,(Select Id from 
SBQQ__Subscriptions__r) from Contract where Id IN: contractIds]){
Contract conObj = new Contract ();
conObj.Id = con.Id;
conObj.Number_of_Subscriptions__c = con.SBQQ__Subscriptions__r.size();
lstContractsToUpdate.add(conObj);
}

UPDATE lstContractsToUpdate;
}    

The code is getting saved without any error.But the field on Contract called: Number of Subscription is not getting any updated.

Answer 1

You are pulling the ids that go into contractIds from a relationship on the trigger.new/old

 for(SBQQ__Subscription__c s:trigger.new){
     contractIds.add(s.SBQQ__Contract__r.id);
 }

Fields pulled from a parent relationship are not populated in trigger context variables.

So instead do:

for(SBQQ__Subscription__c s:trigger.new){
     contractIds.add(s.SBQQ__Contract__c);
 }