one to one salesforce

trigger OneToOneValidate on Passport__c (before insert, before update) {
	Set<id> personIds = new Set<id>();
	Map<id, person__c> mapPerson = new Map<id, person__c>();

	for (Passport__c p : trigger.New) {
		personIds.add(p.Person__c);
	}

	List<Person__c> lstPersons = [SELECT Id, Name FROM Person__c WHERE Id = : personIds];
	if (!lstPersons.isEmpty()) {
		for (Persons__c emp : lstPersons) {
			mapPerson.put(emp.Id, emp);
		}

		for (Passport__c p : trigger.New) {
			if (mapPerson.containsKey(p.Person__c)) {
				p.addError('A passport already exist for the person - ' + mapPerson.get(p.Person__C).Name);
			}
		}
	}
}