var results_Works =
MongoDatabase.Create("someconnstring").GetCollection<Swine>("Swine").Find(Query.EQ("AnimalId", "123456")).ToList();

var results_DoesntWork =
MongoDatabase.Create("someconnstring").GetCollection<Swine>("Swine").Find(Query.EQ("AnimalId", "123456")).SetFields("AnimalId").ToList();