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Khan AnasKhan Anas 

How to write Before Delete Trigger to restrict deletion

I am new to Salesforce, please help me. I have custom object Book__c with name and price__c fields. I need to write a trigger that restricts deletion of a book which is the costliest.
Below is my code: 
Trigger Book_Del_trg on Book__c (before delete) {
    List <Book__c> bag = trigger.old;
    bag = [select id, price__c from Book__c order by price__c desc limit 1];
    for (Book__c book : bag){
        if(book.id != null){
            Book__c b = Trigger.oldMap.get(book.Id); 
            b.addError('Cannot delete costliest book');
        }  
    }
}

When I am deleting Costliest book it is working fine. But when I am trying to delete other books, it is showing following error:

There were custom validation error(s) encountered while saving the affected record(s). The first validation error encountered was "Apex trigger Book_Del_trg caused an unexpected exception, contact your administrator: Book_Del_trg: execution of BeforeDelete caused by: System.NullPointerException: Attempt to de-reference a null object: Trigger.Book_Del_trg: line 7, column 1". 

Any suggestions please? Thanks in advance.
 
Best Answer chosen by Khan Anas
Suraj TripathiSuraj Tripathi
Trigger Book_Del_trg on Book__c (before delete) {
    List <Book__c> bag_old = trigger.old;
    List <Book__c> Bag_new = new List<Book__c>();
    bag_new = [select id, price__c from Book__c order by price__c desc limit 1];
    for (Book__c book_outer : bag_old){
        for(Book__c book_inner : bag_new)
        {
            if(book_inner.id == book_outer.id){
                //Book__c b = Trigger.oldMap.get(book.Id); 
                book_outer.addError('Cannot delete costliest book');
            }
        }
    }
}
You can try This Code it works Fine.

Mark it best if it helps you

Regards
Suraj
 

All Answers

Suraj TripathiSuraj Tripathi
Trigger Book_Del_trg on Book__c (before delete) {
    List <Book__c> bag_old = trigger.old;
    List <Book__c> Bag_new = new List<Book__c>();
    bag_new = [select id, price__c from Book__c order by price__c desc limit 1];
    for (Book__c book_outer : bag_old){
        for(Book__c book_inner : bag_new)
        {
            if(book_inner.id == book_outer.id){
                //Book__c b = Trigger.oldMap.get(book.Id); 
                book_outer.addError('Cannot delete costliest book');
            }
        }
    }
}
You can try This Code it works Fine.

Mark it best if it helps you

Regards
Suraj
 
This was selected as the best answer
Amit Chaudhary 8Amit Chaudhary 8
Please try to update your code like below 
Trigger Book_Del_trg on Book__c (before delete) 
{
    for (Book__c book : trigger.old )
	{
        if(book.id != null && book.price__c  != null)
		{
            b.addError('Cannot delete costliest book');
        }  
    }
}

Let us know if this will help you