如何仅查询 RealmObject 的特定属性？

Question

例如，我有对象：

public class person : RealmObject { 
        public string firstname { get; set; }
        public string secondname { get; set; }
        public int age { get; set; }
        public string address { get; set; }
}

如何进行查询，只给我地址或名字+第二名的列表？

或者名字和年龄。

Answer 1

一般Select目前不支持...

只要您的Select 投影RealmObject，它就受支持，但您要求的投影将类型从RealmObject 更改为另一种类型并且不支持：

• https://github.com/realm/realm-dotnet/issues/916

如果您确实需要断开对象与 Realm 数据库的连接，您可以将基本查询带到一个列表 (.ToList)，然后然后对结果执行 Select 投影：

realm.All<Dog>().ToList().Select(dog => dog.Name);

更新：

对于您的名字+名字问题，我会将其添加为您的 RealmObject 子类的只读属性。由于该属性有一个自定义的 setter/getter，它不会被持久化在 Realm 数据存储中。

public class person : RealmObject
{
    public string firstname { get; set; }
    public string secondname { get; set; }
    public int age { get; set; }
    public string address { get; set; }

    public string name { get { return firstname + " " + secondname; } }
}

然后你可以这样做：

var realm = Realms.Realm.GetInstance("*.db");
var me = new person() { firstname = "Sushi", secondname = "Hangover", age = 99, address = "Local sushi bar" };
realm.Write(() => realm.Add<person>(me));
var user = realm.All<person>().Where((person p) => p.firstname == "Sushi" && p.secondname == "Hangover").FirstOrDefault();
Console.WriteLine($"{user.name} is {user.age} years old and is currently at {user.address}");

输出：

Sushi Hangover is 99 years old and is currently at Local sushi bar

更新2：

如果您希望遍历返回的 RealmObjects 集合：

var users = realm.All<person>().Where((person p) => p.age < 40);
Console.WriteLine($"There are {users.Count()} users under 40 years of age and they are located at:");
foreach (var user in users)
{
    Console.WriteLine($"{user.firstname} is currently at {user.address}");
}

Answer 2

您可以尝试以下方法：

var realm = Realm.GetInstance();

var addresses = realm.All<person>()
                     .ToList()
                     .Select(person => person.address)
                     .ToList();

var firstAndSecondNames = realm.All<person>()
                               .ToList()
                               .Select(person => new
                               {
                                   FirstName = person.firstName,
                                   SecondName = person.secondName
                               })
                               .ToList();