如何在 Android 中填充中间 table

Question

我有两个具有多对多关系的 table。所以我创建了一个中间 table，但我找不到正确填充此 table 的方法，因为我无法根据我的数据设置正确的列表。

我有一个 'courses' 列表：每个 'course' 可以有一个或多个类别。所以我的 table 看起来像这样：

|idcourses|title|date|categories|
|----|----|----|----|
|700|title1|01012021|[54]|
|701|title2|01022021|[54]|
|702|title3|01032021|[48]|
|868|title4|01042021|[47, 52, 54]|

如果我尝试这样的地图：

val myMap = coursesList.map { itcategory to it.idcourses}.distinct()

我有这样的结果:

([54], 700), ([54], 701), ([48], 702), ([47, 52, 54], 868)

整个“[47, 52, 54]”被视为一个字符串，但我希望将其拆分，这样我就可以得到这个：

([54], 700), ([54], 701), ([48], 702), ([47], 868), ([52], 868), ([54], 868)

有人知道如何实现吗？？

Answer 1

val tempMap = mutableMapOf<Int, Int>()
for (course in courseList) {
    course.categories.map {
        tempMap[it] = course.id
    }
}

或

val tempMap = mutableMapOf<List<Int>, Int>()
for (value in courseList) {
    value.categories.map {
        tempMap[listOf(it)] = value.id
    }
}

它不是优化的解决方案，但它的工作

Answer 2

我认为您可能尝试以错误的方式执行此操作，因为看起来您的中间 table 有一列您期望类别 ID 列表。

列不能是 list/array 它必须是单个对象。

但是而不是试图解决这个问题，通常用于中间 table 的是一个 table，它主要有一行映射。即构成映射的两列。其中两列是复合主键。

可以使用具有映射特定数据的其他列。
在你的情况下，一栏 map/reference/relate/associate 到课程，第二栏映射课程。

例如，假设您有课程 Table 和类别 Table 每个：-

@Entity
data class Course(
    @PrimaryKey
    val idcourses: Long? = null,
    val title: String,
    val date: String
)

和

@Entity
data class Category(
    @PrimaryKey
    val idcategories: Long? = null,
    val name: String
)

那么你可以将中间 table 作为 :-

@Entity(primaryKeys = ["idcoursesmap","idcategoriesmap"])
data class CourseCategoryMap(
    val idcoursesmap: Long,
    @ColumnInfo(index = true)
    val idcategoriesmap: Long
)

idcategoriesmap 上的索引可能会提高效率。房间也会发出警告。
您可能希望考虑定义外键约束以强制执行参照完整性。 None 为简洁起见已包括在内。

这对于多对多关系已经足够了。

您可能想要检索带有类别的课程，因此您可能需要一个 POJO，例如：-

data class CourseWithCategories(
    @Embedded
    val course: Course,
    @Relation(
        entity = Category::class,
        parentColumn = "idcourses",
        entityColumn = "idcategories",
        associateBy = Junction(
            value = CourseCategoryMap::class,
            parentColumn = "idcoursesmap",
            entityColumn = "idcategoriesmap"
        )
    )
    val categories: List<Category>
)

这里有一些可能或可能是的道 wanted/useful:-

abstract class AllDao {
    @Insert(onConflict = IGNORE) // Insert single Category
    abstract fun insert(category: Category): Long
    @Insert(onConflict = IGNORE) // Insert Single Course
    abstract fun insert(course: Course): Long
    @Insert(onConflict = IGNORE) // Insert Single CourseCategoryMap
    abstract fun insert(courseCategoryMap: CourseCategoryMap): Long
    /* Inserts many course category maps */
    @Insert(onConflict = IGNORE)
    abstract fun insert(courseCategoryMaps: List<CourseCategoryMap>): List<Long>

    @Query("SELECT * FROM course WHERE course.title=:courseTitle")
    abstract fun getCourseByTitle(courseTitle: String): Course
    @Query("SELECT * FROM category WHERE category.name LIKE :categoryMask")
    abstract fun getCategoriesByNameMask(categoryMask: String): List<Category>
    /* For retrieving courses with all the courses categories */
    @Transaction
    @Query("SELECT * FROM course")
    abstract fun getAllCoursesWithCategories(): List<CourseWithCategories>

    @Transaction
    @Query("")
    fun insertManyCataegoriesForACourseByIds(idcourse: Long,categories: List<Long>) {
        for (categoryId: Long in categories) {
            insert(CourseCategoryMap(idcourse,categoryId))
        }
    }

    // Anoher possibility
    @Transaction
    @Query("")
    fun insertManyCategoriesForACourse(course: Course, categories: List<Category>) {
        val categoryIds = ArrayList<Long>()
        for (c: Category in categories) {
            categoryIds.add(c.idcategories!!)
        }
        insertManyCataegoriesForACourseByIds(course.idcourses!!,categoryIds)
    }
}

演示

为了演示上面的内容，一个非常标准的 class 用 @Database 注释：-

const val DATABASE_NAME = "the_database.db"
const val DATABASE_VERSION =1
@Database(entities = [Course::class,Category::class,CourseCategoryMap::class], exportSchema = false, version = DATABASE_VERSION)
abstract class TheDatabase: RoomDatabase() {
    abstract fun getAllDao(): AllDao

    companion object {
        @Volatile
        private var instance: TheDatabase? = null
        fun getInstance(context: Context): TheDatabase {
            if (instance == null) {
                instance = Room.databaseBuilder(context,TheDatabase::class.java, DATABASE_NAME)
                    .allowMainThreadQueries()
                    .build()
            }
            return instance as TheDatabase
        }
    }
}

和 activity 代码来复制你正在尝试的样子（但两次显示 2 种映射方式，第二次使用比第一种大 20 的类别 ID）:-

lateinit var db: TheDatabase
lateinit var dao: AllDao
override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)

    db = TheDatabase.getInstance(this)
    dao = db.getAllDao()

    dao.insert(Course(idcourses = 700,title = "title1", date = "01012021"))
    dao.insert(Course(701,"title2","01022021"))
    dao.insert(Course(702,"title3","01032021"))
    dao.insert(Course(868,"title4","01042021"))

    // add quite a few categories for demo
    for(i in 30..300) {
        dao.insert(Category(i.toLong(),"Category${i}"))
    }

    //example of what you are trying to do (first)
    var currentCourse = dao.getCourseByTitle("title1")
    dao.insertManyCataegoriesForACourseByIds(currentCourse.idcourses!!, listOf(54))
    dao.insertManyCataegoriesForACourseByIds(dao.getCourseByTitle("title2").idcourses!!, listOf(54))
    dao.insertManyCataegoriesForACourseByIds(dao.getCourseByTitle("title3").idcourses!!, listOf(48))
    dao.insertManyCataegoriesForACourseByIds(dao.getCourseByTitle("title4").idcourses!!, listOf(47,52,54))

    // second (does the same but uses categroyids 20 greater than the first)
    val coursecategorymaplist = listOf<CourseCategoryMap>(
        CourseCategoryMap(700,74),
        CourseCategoryMap(701,74),
        CourseCategoryMap(702,68),
        CourseCategoryMap(868,67),
        CourseCategoryMap(868,72),
        CourseCategoryMap(868,74)
    )
    dao.insert(coursecategorymaplist)

    // Extract results
    for (cwc: CourseWithCategories in dao.getAllCoursesWithCategories()) {
        Log.d("DBINFO","Course is ${cwc.course.title}, date is ${cwc.course.date} it has ${cwc.categories.size} categories they are:-")
        for (c: Category in cwc.categories) {
            Log.d("DBINFO","\tCategory is ${c.name}")
        }
    }
}

结果

日志包括（注意类别数量的两倍）：-

D/DBINFO: Course is title1, date is 01012021 it has 2 categories they are:-
D/DBINFO:   Category is Category54
D/DBINFO:   Category is Category74
D/DBINFO: Course is title2, date is 01022021 it has 2 categories they are:-
D/DBINFO:   Category is Category54
D/DBINFO:   Category is Category74
D/DBINFO: Course is title3, date is 01032021 it has 2 categories they are:-
D/DBINFO:   Category is Category48
D/DBINFO:   Category is Category68
D/DBINFO: Course is title4, date is 01042021 it has 6 categories they are:-
D/DBINFO:   Category is Category47
D/DBINFO:   Category is Category52
D/DBINFO:   Category is Category54
D/DBINFO:   Category is Category67
D/DBINFO:   Category is Category72
D/DBINFO:   Category is Category74

数据库

课程Table:-

类别Table（部分）

CourseCategoryMap（中级table）

如何在 Android 中填充中间 table

how to populate intermediate table in Android

android

kotlin

android-room