请教个 android room 查询问题

data class AreaAndMedia(
    @Embedded
    val area: Area,
    @Relation(parentColumn = "objId" , entityColumn = "bindArea")
    var medias: List<Media>
)

data class Media(
	@ColumnInfo(name = "taskId", defaultValue = "")
    val taskId: String = ""
)

interface Dao {
	@Transaction
    @Query("select * from area where objId = :areaId")
    fun selectAreaAndMediasById(areaId: String): 	Flow<List<AreaAndMedia>>
}

现在，我想新增一个查询方法，用意是过滤 Media ，例如

@Transaction
    @Query(
        "SELECT af.* FROM area af WHERE af.objId = :areaId AND EXISTS ( SELECT 1 FROM medias mi WHERE mi.bindArea = af.objId AND mi.taskId = '')"
    )
    fun selectAreaAndMediasWithEmptyTaskId(
        areaId: String
    ): Flow<List<AreaAndMedia>>

但是这个结果中，仍然包含 taskId 不为空的 Media, 不知道问题出在哪里