I have a query as below:
WOD_2__Claim__c clm=[select id ,name,DTC_EngineSerialNo__c,DTC_Error__c, DTC_Engine_Mismatch__c, WOD_2__Inventory__r.Engine_Serial_Number__c from WOD_2__Claim__c where id=:claimid limit 1];
However when I debug clm.WOD_2__Inventory__r.Engine_Serial_Number__c is returning NULL.

The code runs in System mode. May I know why is this happeing
 

Can anybody help to out of this que how to associated record and count display in oppoprtunity in trigger.I just started to learn development.