android - Android Room 中的实体列表关系

Question

我正在尝试加载实体子列表，但我想避免进行 2 次查询。

我正在考虑在 TypeConverter 中进行查询，但我真的不知道这是否是个好主意。

我的实体:

@Entity
class Region(
        @PrimaryKey(autoGenerate = true)
        var id: Int = 0,
        var name: String = "",
        var locales: List<Locale> = listOf())

@Entity(foreignKeys = arrayOf(ForeignKey(
        entity = Region::class,
        parentColumns = arrayOf("id"),
        childColumns = arrayOf("regionId"),
        onDelete = CASCADE,
        onUpdate = CASCADE
)))
class Locale(
        @PrimaryKey(autoGenerate = true)
        var id: Int = 0,
        var regionId: Int = 0,
        var name: String = "")

DAO:

@Dao
interface RoomRegionDao{
    @Insert
    fun insert(region: Region)

    @Delete
    fun delete(region: Region)

    @Query("select * from region")
    fun selectAll(): Flowable<List<Region>>
}

@Dao
interface RoomLocaleDao{
    @Insert
    fun insert(locale: Locale)

    @Query("select * from locale where regionId = :arg0")
    fun selectAll(regionId: Int): List<Locale>
}

数据库:

@Database(entities = arrayOf(Region::class, Locale::class), version = 1)
@TypeConverters(RoomAppDatabase.Converters::class)
abstract class RoomAppDatabase : RoomDatabase() {
    abstract fun regionDao(): RoomRegionDao
    abstract fun localeDao(): RoomLocaleDao

    inner class Converters {
        @TypeConverter
        fun toLocales(regionId: Int): List<Locale> {
            return localeDao().selectAll(regionId)
        }

        @TypeConverter
        fun fromLocales(locales: List<Locale>?): Int {
            locales ?: return 0
            if (locales.isEmpty()) return 0

            return locales.first().regionId
        }
    }
}

它不起作用，因为不能将内部类用作转换器类。

• 这是个好方法吗？
• 当我执行 RoomRegionDao.selectAll 时，如何在区域实体中自动加载“语言环境列表”？

Answer 1

我认为 TypeConverter仅适用于静态方法。 我是根据 here 中的示例说的和 here

来自关系部分 here :

"Because SQLite is a relational database, you can specify relationships between objects. Even though most ORM libraries allow entity objects to reference each other, Room explicitly forbids this."

所以我想最好是添加 @Ignore在你的 locales属性并在 RoomLocaleDao 上创建方法那就是插入 List<Locale>插入Region后调用。

插入区域的方法可以返回inserted id .

If the @Insert method receives only 1 parameter, it can return a long, which is the new rowId for the inserted item. If the parameter is an array or a collection, it should return long[] or List instead. (https://developer.android.com/topic/libraries/architecture/room.html#daos-convenience)