如何从多个实体中选择带有房间数据库的新对象？

Question

我有这个tow pojos for RoomDB实体，

@Entity
public class Favorite {...}

@Entity
public class ProductCart {
    Integer productQuantity;
    ...
}

我需要应用此选择 statment，然后将其返回对象fairitiTeadapterData

SELECT favorite.*,
       (SELECT CASE count(productQuantity)
               WHEN 0 THEN 0
                      ELSE productQuantity
               END  FROM ProductCart 
        WHERE favorite.id = productId) AS cartQuantity
FROM Favorite favorite

sqlite Statment Test

public class FavoriteAdapterData {
    int cartQuantity = 0;
    Favorite favorite;
    ...
}

我尝试以下代码，

@Query("SELECT  " +
            "      (    SELECT CASE count(productQuantity) " +
            "           WHEN 0  THEN 0 " +
            "                   ELSE productQuantity  " +
            "           END  " +
            "           FROM ProductCart " +
            "           WHERE productId = favorite.id" +
            "       ) AS cartQuantity" +
            "       ,favorite.* " +
            "FROM Favorite favorite")
    LiveData<List<FavoriteAdapterData>> getFavoriteProducts();

但它不起作用，Android Studio notfi我这个错误：

Cannot figure out how to read this field from a cursor. 
Favorite favorite; 
             ^

Answer 1

您需要使用 @embedded 注释，它将从喜欢的类的字段/变量中确定列和输出字段，

例如： -

public class FavoriteAdapterData {
    int cartQuantity = 0;
    @Embedded
    Favorite favorite;
    ...
}