Apex - Trigger.old and trigger.new comparison

trigger contacttrigger on contact (before delete) {
    for (customobject__C coj : [SELECT Contactlookup__c
                                      FROM contactrelatedobject__c
                                      WHERE Contactlookup__c
                                      IN :Trigger.oldMap.keySet()]) {
        Trigger.oldMap.get(coj.contactlookup__c).addError(
                   'Cannot delete contact with a contactrelatedobject record');
    }
}