Trigger to check Check-box field

Question

I have Two objects. In both objects I have Holdback field whose datatype is check-box. Whenever I check or uncheck field of Object 1, field of Object 2 should check or uncheck respectively. But field from Object 2 should be read-only. I want a trigger for same.

trigger CheckHoldBack on dealer__Sales_Up__c (after insert,after update) {

    List<dealer__Vehicle_Inventory__c> objSalesUp =[SELECT Id, Name, dealer__Holdback__c FROM dealer__Vehicle_Inventory__c ];
    for(dealer__Sales_Up__c s: Trigger.new){
        System.debug('***********HoldBack********'+s.Holdback__c);    
        if(s.Holdback__c == true){
            System.debug('********HoldBack********'+s.Holdback__c);
            s.dealer__Desired_Stock_Num__r.Holdback__c = true;
        }
        else{
            s.dealer__Desired_Stock_Num__r.Holdback__c = false;
        }

    }
}

Answer 1

Based on my understanding of your problem, you want to check the Holdback field on the parent if the same field on the child is checked? So basically you just need to add the parent record with the updated value in a List or Set and make an update on it.

trigger CheckHoldBack on dealer__Sales_Up__c (after insert,after update) {

    List<dealer__Vehicle_Inventory__c> vehicleInventorySet= 
                  new List<dealer__Vehicle_Inventory__c>();

    for(dealer__Sales_Up__c s: Trigger.new){

    dealer__Vehicle_Inventory__c temp = 
            new dealer__Vehicle_Inventory__c(id=s.dealer__Desired_Stock_Num__c,
                                             Holdback__c=s.Holdback__c);        
    vehicleInventorySet.add(temp);  

    }


   update vehicleInventorySet;
}

Edit

For making the field readonly, you would have to do it on a PageLayout level, since basically you need the user to have profile level permissions to edit it.