Account that has related contacts should not be deleted

Question

I need to write a trigger in which if an account that has related contacts and the user tries to delete that account it throws you an error..

trigger DoNotDeleteAccountHavingRelatedContact on Account (before insert, before update)  
{  
    List<Account> accList = new List<Account>();  
    Set<id> accIdSet = new Set<id>();  
    for(Account acc : Trigger.new)  
    {  
        accIdSet.add(acc.id);  
    }  

Map<Id,Contact> mapContacts = new Map<Id,Contact>([Select lastname,id,accountid 
                                        From Contact Where Accountid 
                                        IN : accIdSet]);

for(Account acc : Trigger.new)
{
    if(mapContacts.get(acc.id).contacts.size()>0)
    {
        acc.adderror('Account cannot be deleted');
    }
}                                       

}

Answer 1

Your trigger is a Before Insert/Update. Try:

trigger DoNotDeleteAccountHavingRelatedContact on Account (before delete)

And do replace Trigger.new with Trigger.old

** UPDATE **

Another commenter raised the issue that there was also a code problem. Please find an updated sample below

trigger DoNotDeleteAccountHavingRelatedContact on Account (before delete)  
{  
    List<Account> accList = new List<Account>();  
    Set<id> accIdSet = new Set<id>();  
    for(Account acc : Trigger.old)  
    {  
        accIdSet.add(acc.id);  
    }  

    Map<Id, Account> accts = new Map<Id, Account>([Select Id, (Select Id from contacts) from Account where id in :accIdSet]);

    for(Account acc : Trigger.old)
    {
        if(accts.get(acc.id).contacts.size()>0)
        {
            acc.adderror('Account cannot be deleted');
            }
        }                                       

}

Answer 2

Unless I am wrong, the query will fill your map with the Contact.id => Contact not with the AccountId. So the get call on the map will return null.

Here is what I would do:

trigger DoNotDeleteAccountsWithContacts on Account(before delete) {
Set<Id> accountsWithContacts = new Set<Id>();
for(Contact c : [SELECT Id, AccountId FROM Contact WHERE AccountId in :trigger.oldMap.keySet()]) {
    accountsWithContacts.add(c.AccountId);
}
for(Id id : accountsWithContacts) {
    trigger.oldMap.get(id).addError('Account cannot be deleted while there are related contacts');
}

}

Answer 3

You could also query for accounts that have contact records and prevent those from being deleted like:

trigger DoNotDeleteAccountHavingRelatedContact on Account (before delete)  
{  
    for(Account acc : [SELECT Id FROM Account WHERE Id IN (SELECT AccountId FROM Contact) AND Id IN :Trigger.oldMap.KeySet()]){
        Trigger.oldMap.get(acc.Id).adderror('Account cannot be deleted');
    }
}