Room: Relation entities using Room,(One to many) 检索一个有条件而不是所有元素

Question

我的问题与 Android Room: Insert relation entities using Room 相似但变化不大

@Entity
public class Pet {
    @PrimaryKey
    public int id;     // Pet id
    public int userId; // User id
    public String name;
    //Added EXTRA column

    public long time;// pet since have
}

目前 UserWithPets POJO 是

// Note: No annotation required at this class definition.
public class UserWithPets {
   @Embedded
   public User user;

   @Relation(parentColumn = "id", entityColumn = "userId", entity = Pet.class)
   public List<Pet> pets;
}

查询是

    @Query("SELECT * FROM User")
    public List<UserWithPets> loadUsersWithPets();

但我需要这样的东西 UserWithLatestPet POJO：

public class UserWithLatestPet {
   @Embedded
   public User user;

   @Relation(parentColumn = "id", entityColumn = "userId", entity = Pet.class)
   public Pet latestPet;//Retrieve only latest (since time) pet per user
}

//Query
@Query("SELECT * FROM User")
public List<UserWithLatestPet> loadUserWithLatestPet();

仅查询来自宠物实体和用户的最新宠物的最佳方式是什么

Answer 1

首先，您只能将单个 Pet 定义为 Relation 的结果，因为 Room Version 2.2.0-alpha01

我认为你可以做两件事： 1.创建查询左加入需要的宠物 2.创建正确的关系List<Pet>然后扫描最新的List。

如果你想节省一点，你可以创建一个只包含宠物列表的时间和 ID 的 POJO，这样你就只会得到你需要的最少数据，扫描列表找到最新的然后查询为了那只宠物

Room: Relation entities using Room,(One to many) 检索一个有条件而不是所有元素

Room: Relation entities using Room,(One to many) Retrieve one with condition rather than all elements

android

android-sqlite

android-database

android-room