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我有一个 UI 模型CourseUiModel，我在 ViewModel 中使用它。

data class CourseUiModel(
        val Id: String,
        val Title: String,
        val Subtitle: String,
        // Author is missing
        val lessonModels: List<LessonUiModel>,
)

data class LessonUiModel(
        val Id: String,
        val Name: String,
        val topicModels: List<TopicUiModel>,
)

data class TopicUiModel(
        val Id: String,
        val Name: String
)

我在本地数据库上有 3 个表，分别称为Course、Lesson和Topic。实体类是：

    @Entity
    @Parcelize
    data class Course(
            @PrimaryKey
            val Id: String,
            val Title: String,
            val Subtitle: String,
            val Author: String,
    ) : Parcelable {}
    
    @Entity(indices = [Index(value = ["CourseId"])],
            foreignKeys = [ForeignKey(
                    entity = Course::class,
                    parentColumns = arrayOf("Id"),
                    childColumns = arrayOf("CourseId"),
                    onDelete = ForeignKey.CASCADE)])
    @Parcelize
    data class Lesson(
            @PrimaryKey
            val Id: String,
            val Name: String,
            val CourseId: String,
            val OrderInCourse: Int,
    ) : Parcelable {}
    
    @Entity(indices = [Index(value = ["LessonId"])],
            foreignKeys = [ForeignKey(
                    entity = Lesson::class,
                    parentColumns = arrayOf("Id"),
                    childColumns = arrayOf("LessonId"),
                    onDelete = ForeignKey.CASCADE)])
    @Parcelize
    data class Topic(
            @PrimaryKey
            val Id: String,
            val Name: String,
            val LessonId: String,
            val OrderInLesson: Int,
    ) : Parcelable {}

如何将我从房间得到的查询映射到这个对象？我目前拥有的是以下内容：

@Query("""SELECT *
    FROM Course
    JOIN Lesson ON Course.Id = Lesson.CourseId
    JOIN Topic ON Lesson.Id = Topic.LessonId
    WHERE Course.Id = courseId
    ORDER BY Lesson.OrderInCourse, Topic.OrderInLesson ASC""")
fun getCourseModel(courseId: String): LiveData<CourseUiModel>

当我尝试运行它时，我收到以下错误

error: The columns returned by the query does not have the fields [lessonModels] in com.albiders.albiders.ui.Courses.ui_models.CourseUiModel even though they are annotated as non-null or primitive. Columns returned by the query: [Id,Title,Subtitle,Author,Id,Name,CourseId,OrderInCourse,Id,Name,LessonId,OrderInLesson]

该错误是有道理的，但我不知道如何正确执行此操作。

4

1 回答 1

1

我相信你想用它@Relation来构建数组

CourseUiModel可以是：-

data class CourseUiModel(
    val Id: String,
    val Title: String,
    val Subtitle: String,
    // Author is missing

    @Relation(entity = Lesson::class,entityColumn = "CourseId",parentColumn = "Id")
    val lessonModels: List<LessonUiModel>
)

和

LessonUiModel可以是：-

data class LessonUiModel(
    val Id: String,
    val Name: String,
    @Relation(entity = Topic::class,entityColumn = "LessonId",parentColumn = "Id")
    val topicModels: List<TopicUiModel>
)

但是，我相信以上不会产生您想要的结果。这是因为 ORDER BY 子句（课程顺序和主题顺序）不会被视为 Room，我相信它从最顶层的父（课程）对象构建子对象，因此 ORDER BY 仅与最顶层的对象相关（课程）。

事实上，你可以使用("SELECT * FROM Course WHERE courseId = :courseID:)它，它会提取相同的数据。

于 2021-04-10T02:11:00.240 回答