Question

我正在使用Android中的Room作为数据库，也正在使用@Relation（基于此：https://developer.android.com/reference/android/arch/persistence/room/Relation.html）从与ForeinKey一对多相关的关系实体中获取数据。我想要得到的是使用@Relation从RoomArea实体中获得带有roomAreaNames的列表。该代码没有编译错误，问题在于我只是从@Relation回来的是一个大小为1的列表（仅是表中的第一个对象），而不是完整列表。

表格：

@Entity(
    tableName = "buildings_table",
    indices = [Index("contract_id")],
    foreignKeys = [
        ForeignKey(
            entity = Contract::class,
            parentColumns = ["contract_id"],
            childColumns = ["contract_id"],
            onDelete = ForeignKey.CASCADE)]
)
data class Building(
    @PrimaryKey(autoGenerate = true) @ColumnInfo(name = "building_id")
    val buildingId: Long = 0L,

    @ColumnInfo(name = "contract_id")
    val contractId: Long,

    @ColumnInfo(name = "building_name")
    val buildingName: String)

@Entity(
    tableName = "floors_table",
    indices = [Index("building_id")],
    foreignKeys = [
        ForeignKey(
            entity = Building::class,
            parentColumns = ["building_id"],
            childColumns = ["building_id"],
            onDelete = ForeignKey.CASCADE)]
)
data class Floor(
    @PrimaryKey(autoGenerate = true) @ColumnInfo(name = "floor_id")
    val floor_id: Long = 0L,

    @ColumnInfo(name = "building_id")
    val buildingId: Long,

    @ColumnInfo(name = "level")
    val level: Int
)

@Entity(
    tableName = "rooms_area_table",
    indices = [Index("floor_id")],
    foreignKeys = [
        ForeignKey(
            entity = Floor::class,
            parentColumns = ["floor_id"],
            childColumns = ["floor_id"],
            onDelete = ForeignKey.CASCADE)]
)
data class RoomArea(
    @PrimaryKey(autoGenerate = true) @ColumnInfo(name = "room_area_id")
    val roomAreaId: Long = 0L,

    @ColumnInfo(name = "floor_id")
    val floorId: Long,

    @ColumnInfo(name = "room_area_name")
    val roomAreaName: String
)

Dao查询：

@Transaction
@Query("SELECT * FROM buildings_table WHERE contract_id = :contractId")
fun getItemsAuditBuilding(contractId: Long): LiveData<List<ItemAuditBuilding>>

这里是@Relation（仅给我一个列表大小1），我需要与buildingId相关的所有roomAreas

data class ItemAuditBuilding(

    @Embedded val building: Building,
    @Relation(
        parentColumn = "building_id",
        entityColumn = "room_area_id",
        entity = RoomArea::class,
        projection = ["room_area_name"]
    )
    var roomAreas: List<String>
)

谢谢。

Answer 1

您正在尝试获取字符串列表。我认为您实际上打算这样做

data class ItemAuditBuilding(
@Embedded val building: Building,
@Relation(
    parentColumn = "building_id",
    entityColumn = "room_area_id",
    entity = RoomArea::class,
    projection = ["room_area_name"]
)
var roomAreas: List<RoomArea>

）

Answer 2

我希望这对尝试使用Room关系的人有所帮助。我已根据2019年Android Dev Summit提出了我的问题的答案。以下是Florina谈到房间的部分的链接：[https://www.youtube.com/watch?v=_aJsh6P00c0]。根据视频，我进行了以下更改：

我添加了一个Junction表：

@Entity(
    primaryKeys = ["building_id", "room_area_id"],
    indices = [Index("room_area_id")]
)
data class RoomAreaWithBuilding(
    @ColumnInfo(name = "building_id")
    val buildingId: Long,
    @ColumnInfo(name = "room_area_id")
    val roomAreaId: Long
)

然后我修改了ItemAuditBuilding类

data class ItemAuditBuilding(

    @Embedded val building: Building,

    @Relation(
        parentColumn = "building_id",
        entity = RoomArea::class,
        entityColumn = "room_area_id",
        projection = ["room_area_name"],
        associateBy = Junction(RoomAreaWithBuilding::class)
    )
    val roomAreas: List<String>
)

注意：要使此代码正常工作，您需要使用Room 2.2，因为 associateBy 是一项新功能。享受吧！

Room @Relation仅返回表的第一项

2 个答案: