假设我有两个实体,问题和答案,它们具有一对多关系,每个问题可能有 0..n 个答案。它们定义为:
@Entity(tableName="questions")
data class Question (
@PrimaryKey
val id: Long,
val questionText: String
)
@Entity(tableName="answers")
data class Answer (
@PrimaryKey
val id: Long,
val questionId: Long,
val userId: Long,
val answerText: String
)
data class QuestWithAns(
@Embeded
val question: Question,
@Relation(parentColumn = "id", entityColumn = "questionId", entity = Answer::class)
val answer: Answer?
)
我想查询数据库并获取列表QuestWithAns
包含所有问题且只有一个答案的对象userId
如果该用户尚未回答问题,则返回 null。我尝试在 DAO 对象中实现此查询,如下所示:
@Dao
abstract class QuestionDAO {
@Query("SELECT * FROM questions AS quest LEFT OUTER JOIN (SELECT * FROM answers WHERE userid=:userId) AS ans ON quest.id = ans.questionId")
abstract fun getQuestWithAnsByUser(userId: Long): LiveData<List<QuestWithAns>>
}
但对于该用户尚未回答的问题,room 将返回任何其他用户的最后一个答案,而不是 null。
谁能帮我找出我做错了什么?有没有办法用@Relation来实现这个查询?