房间数据库中的一对多关系

Question

我的项目中有一个事件，这个事件中有时间表。每个事件都有自己的时间表。我想用我写的代码列出每个事件的时间线，但是我无法通过适配器访问它。

我的实体；

事件：

@Parcelize
@Entity(tableName = "event_table")
data class Event(
    @PrimaryKey(autoGenerate = true)
    val eventId: Int,
    val eventName: String,
    val eventCategory: String
): Parcelable

时间线：

   @Parcelize
    @Entity(tableName = "timeline_table")
    data class Timeline(
        @PrimaryKey(autoGenerate = true)
        val TimeLineId: Int,
        val TimeLineAccomplish: String,
        val TimeLineDetails: String,
        val TimeLineDate: String,
        val eventCreatorId: Int
    ): Parcelable

事件和时间线：

data class EventAndTimeline(
    @Embedded val event: Event,
    @Relation(
        parentColumn = "eventId",
        entityColumn = "eventCreatorId"
    )
    val timeline: List<Timeline>
)

数据库：

@Database(entities = [Timeline::class, Event::class], version = 1, exportSchema = false)
abstract class TimeLineDatabase: RoomDatabase() {
    abstract fun timelineDao(): TimelineDao
    abstract fun eventDao(): EventDao

    companion object{
        @Volatile
        private var INSTANCE: TimeLineDatabase? = null

        fun getDatabase(context: Context): TimeLineDatabase {
            val tempInstance = INSTANCE
            if (tempInstance != null) {
                return tempInstance
            }
            synchronized(this) {
                val instance = Room.databaseBuilder(
                    context.applicationContext,
                    TimeLineDatabase::class.java,
                    "timeline_database"
                )
                    .build()

                INSTANCE = instance
                return instance
            }
        }
    }
}

道：

@Dao
interface TimelineDao{
    @Transaction
    @Query("SELECT * FROM event_table")
    fun getEventsWithTimelines(): LiveData<List<EventAndTimeline>>
}

存储库：

class TimelineRepository(private val timelineDao: TimelineDao) {
   
    val readAllData: LiveData<List<EventAndTimeline>> = timelineDao.getEventsWithTimelines()
}

视图模型：

class TimeLineViewModel(application: Application): AndroidViewModel(application) {

    val readAllData: LiveData<List<EventAndTimeline>>
  
    private val timelineRepository: TimelineRepository

    init {
        val timelineDao = TimeLineDatabase.getDatabase(application).timelineDao()
        timelineRepository = TimelineRepository(timelineDao)
        readAllData = timelineRepository.readAllData
    }

最后：在我的列表适配器中，我想列出事件的时间表。当我没有关系时，我会到达所有时间线：

holder.itemView.findViewById<TextView>(R.id.timelineDetails_txt).text 
currentItem.TimeLineDetails

但现在我看不到时间线变量（如 TimeLineDetails、TimeLineAccomplish 等）。

Answer 1

您需要像下面的示例一样编写查询。

  interface UserBookDao {
        @Query(
            "SELECT user.name AS userName, book.name AS bookName " +
            "FROM user, book " +
            "WHERE user.id = book.user_id"
        )
        fun loadUserAndBookNames(): LiveData<List<UserBook>>
    
        // You can also define this class in a separate file.
        data class UserBook(val userName: String?, val bookName: String?)
    }

您可以在下面查看更多示例 link。

https://developer.android.com/training/data-storage/room/relationships

房间数据库中的一对多关系

One to many relationship in Room Database

android

kotlin

android-room

one-to-many

android-room-relation