查詢1：AndroidRoom中的許多關系

我在我的 Android 應用程式 (Java) 中使用 Room 并且我有兩個具有 1:many 關系的物體。

鏡片物體 一個鏡片可以多次佩戴。

@Entity(tableName = "lens_table")
public class Lens {

    @PrimaryKey(autoGenerate = true)
    private int lensId;
    private String name;
}

佩戴物體 一次佩戴只能與一個鏡片相關。

@Entity(tableName = "wear_table",
        foreignKeys = {@ForeignKey(
                entity = Lens.class,
                parentColumns = "lensId",
                childColumns = "fk_lensId",
                onDelete = ForeignKey.CASCADE)},
        indices = {@Index("fk_lensId")})
public class Wear {

    @PrimaryKey(autoGenerate = true)
    private int wearId;
    private String name;
    private int fk_lensId;
}

到現在為止還挺好。到目前為止，我對“標準”查詢（創建、獲取全部、更新、洗掉等）感到滿意，其中有很多檔案。我還成功實施了查詢，以根據以下關系獲取所有鏡片的佩戴情況。

public class LensWithWears {

    @Embedded
    public Lens lens;

    @Relation(
            parentColumn = "lensId",
            entityColumn = "fk_lensId"
    )
    public List<Wear> wears;
}

但現在我需要查詢以下資訊：

通過查找wearId 獲取關聯鏡片的單次佩戴

我目前使用的關系類如下所示：

public class WearWithLens {
    @Embedded
    public Wear wear;

    @Relation(
            parentColumn = "wearId",
            entityColumn = "lensId"
    )
    public Lens lens;
}

Dao Query 看起來像這樣：

@Query("SELECT * FROM wear_table WHERE wearId = :wearId LIMIT 1")
LiveData<WearWithLens> getWearWithLensByWearId(int wearId);

我的代碼顯然不起作用，否則我不會問......問題是，回傳了一個物件WearWithLens，但其中的鏡頭物件始終為空。

換句話說，我想查詢一個 Wear，它與一個鏡頭有 1:1 的關系，并將兩個物件放在 WearWithLens 類中。

有人可以告訴我查詢應該是什么樣子嗎？

謝謝！

uj5u.com熱心網友回復：

父列必須是形成兩者關系的列。也就是說它應該是fk_lensId列。

所以 ：-

public class WearWithLens {
    @Embedded
    public Wear wear;

    @Relation(
            parentColumn = "fk_lensId",
            entityColumn = "lensId"
    )
    public Lens lens;
}

舉個例子

• 不是使用LiveData<WearWithLens> getWearWithLensByWearId(int wearId);而是WearWithLens getWearWithLensByWearId(int wearId);為了方便和簡潔。
• 使用添加了 getter 和 setter 的類/物體以及額外的建構式來減少編碼
• 顯然是上面的@Relationship。

具有以下內容：-

    db = TheDatabase.getInstance(this);
    dao = db.getAllDao();

    int l1id = (int) dao.insert(new Lens("Lens1"));
    int l2id = (int) dao.insert(new Lens("Lens2"));
    int l3id = (int) dao.insert(new Lens("Lens3"));

    dao.insert(new Wear("Wear1 child of Lens1",l1id));
    dao.insert(new Wear("Wear2 child of Lens1",l1id));
    dao.insert(new Wear("Wear3 child of Lens1",l1id));
    dao.insert(new Wear("Wear4 child of Lens2",l2id));
    dao.insert(new Wear("Wear5 child of Lens2",l2id));
    dao.insert(new Wear("Wear6 child of Lens2",l2id));
    dao.insert(new Wear("Wear7 child of Lens3",l3id));

    for (Wear wear: dao.getAllWears()) {
        WearWithLens currentWearWithLens = dao.getWearWithLensByWearId(wear.getWearId());
        Log.d("DBINFO","Current Wear is "   currentWearWithLens.wear.getName()   " parent Lens is "   currentWearWithLens.lens.getName());
    }

結果是：-

2021-12-09 06:34:58.105 D/DBINFO: Current Wear is Wear1 child of Lens1 parent Lens is Lens1
2021-12-09 06:34:58.110 D/DBINFO: Current Wear is Wear2 child of Lens1 parent Lens is Lens1
2021-12-09 06:34:58.112 D/DBINFO: Current Wear is Wear3 child of Lens1 parent Lens is Lens1
2021-12-09 06:34:58.114 D/DBINFO: Current Wear is Wear4 child of Lens2 parent Lens is Lens2
2021-12-09 06:34:58.115 D/DBINFO: Current Wear is Wear5 child of Lens2 parent Lens is Lens2
2021-12-09 06:34:58.116 D/DBINFO: Current Wear is Wear6 child of Lens2 parent Lens is Lens2
2021-12-09 06:34:58.120 D/DBINFO: Current Wear is Wear7 child of Lens3 parent Lens is Lens3

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