android - 在房间数据库android中查询时合并两个实体

Question

假设我们有 User 和 Order 实体

@Entity
public class User{
   int id;
   String name;
}
@Entity
public class Order{
   int id;
   int userId;
   String meal;
}

所以当我查询

@Query("SELECT * FROM users")
public List<User> loadUsers(); 

我想要这个输出:

{"user":{"id":"","name":"",
"order":{"id":"","userId":"","meal":""}}}

Answer 1

您可以对这个问题使用 Relation 注释。注意，官方文档说 [ official reference ]:

The type of the field annotated with Relation must be a List or Set.

@Entity
public class User{
   @PrimaryKey
   int id;
   String name;
   @Relation(parentColumn = "id", entityColumn = "orderId", entity = Order.class)
   List<Order> orders;
}

@Entity
public class Order{
   @PrimaryKey
   int id;
   int userId;
   String meal;
}