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