Update related record with batch class

Question

I'm running a batch class over the Timesheet object and for the timesheets that meet the criteria, I want to update the related Contact record.

I have a jstcl__TG_Timesheet__c object with a field jstcl__Consultant__c relating jstcl__TG_Timesheet__c to the Contact object. I'm trying to run a batch class over the timesheets and for the ones that meet the criteria, I want to update the Previous_Timesheet_Status__c field on the related Contact record. What I have now runs successfully, however it doesn't actually update the field.

public class batchClass implements Database.Batchable<sObject>{
   public Database.QueryLocator start(Database.BatchableContext bc) {
      String query = 'SELECT Id, jstcl__Consultant__r.Previous_Timesheet_Status__c FROM jstcl__TG_Timesheet__c WHERE jstcl__Placement__r.ts2__Status__c =\'Active\' AND jstcl__Placement__r.VMS__c = \'\' AND jstcl__Week_Ending__c = LAST_N_DAYS:4 AND jstcl__Status__c = \'Pending\'';
      return Database.getQueryLocator(query);
    }
   public void execute(Database.BatchableContext BC, List<jstcl__TG_Timesheet__c> a){
      for(jstcl__TG_Timesheet__c b : a){
         b.jstcl__Consultant__r.Previous_Timesheet_Status__c = 'Pending';
      }
    update a;
   }
   public void finish(Database.BatchableContext BC){}
}

Answer 1

The issue here is that your update statement is updating the list of timesheet records, whereas you need it to update a list of contacts. I would rewrite your code as follows:

public class batchClass implements Database.Batchable<sObject>{
   public Database.QueryLocator start(Database.BatchableContext bc) {
      String query = 'SELECT Id, jstcl__Consultant__r.Previous_Timesheet_Status__c FROM jstcl__TG_Timesheet__c WHERE jstcl__Placement__r.ts2__Status__c =\'Active\' AND jstcl__Placement__r.VMS__c = \'\' AND jstcl__Week_Ending__c = LAST_N_DAYS:4 AND jstcl__Status__c = \'Pending\'';
      return Database.getQueryLocator(query);
    }
   public void execute(Database.BatchableContext BC, List<jstcl__TG_Timesheet__c> timesheets){
      List<Contact> consultants = new List<Contact>();
      for(jstcl__TG_Timesheet__c timesheet : timesheets){
         Contact consultant = timesheet.jstcl__Consultant__r;
         if (consultant == null) continue;
         consultant.Previous_Timesheet_Status__c = 'Pending';
         consultants.add(consultant);
      }
      update consultants;
   }
   public void finish(Database.BatchableContext BC){}
}

I took the liberty of assigning some more intuitive variable names than 'a' and 'b'; this can help other people reading your code more easily understand what it does.