要使用Android Room进行子查询,您可以按照以下步骤操作:
build.gradle
文件中,确保您已添加了适当的依赖项:implementation 'androidx.room:room-runtime:2.3.0'
annotationProcessor 'androidx.room:room-compiler:2.3.0'
User
和Post
,并且User
类具有一个userId
字段,而Post
类具有一个userId
字段。在UserDao
接口中,您可以创建一个返回UserWithPosts
对象的方法,该对象包含User
和相关的Post
列表。@Entity(tableName = "users")
public class User {
@PrimaryKey
public int userId;
public String name;
}
@Entity(tableName = "posts")
public class Post {
@PrimaryKey
public int postId;
public int userId;
public String title;
}
public class UserWithPosts {
@Embedded
public User user;
@Relation(parentColumn = "userId", entityColumn = "userId", entity = Post.class)
public List posts;
}
@Dao
public interface UserDao {
@Transaction
@Query("SELECT * FROM users")
List getUsersWithPosts();
}
@Database(entities = {User.class, Post.class}, version = 1)
public abstract class AppDatabase extends RoomDatabase {
public abstract UserDao userDao();
private static volatile AppDatabase INSTANCE;
public static AppDatabase getDatabase(final Context context) {
if (INSTANCE == null) {
synchronized (AppDatabase.class) {
if (INSTANCE == null) {
INSTANCE = Room.databaseBuilder(context.getApplicationContext(),
AppDatabase.class, "app_database")
.build();
}
}
}
return INSTANCE;
}
}
AppDatabase db = AppDatabase.getDatabase(getApplicationContext());
List usersWithPosts = db.userDao().getUsersWithPosts();
这将返回一个List
,其中每个UserWithPosts
对象都包含一个User
对象和与该用户相关的Post
列表。
请注意,上述代码中的版本号、表名和字段名称可能需要根据您的实际情况进行调整。