La til funksjon Book.get_persons.

python/web/library/models.py

@@ -123,6 +123,11 @@ class Book(models.Model):
 				authors.append(person.person)
 		return authors
 
+	def get_persons(self, relation):
+		return map(lambda bp: bp.person,
+			   filter(lambda bp: bp.relation.name == relation,
+				  self.persons.all()))
+
 	def __unicode__(self):
 		return self.title