Android Room 不允许我多次建立一对 N 关系答案

【问题标题】：Android Room doesn't allow me to do the one-to-N relationship multiple timesAndroid Room 不允许我多次建立一对 N 关系
【发布时间】：2021-08-27 12:31:18
【问题描述】：

我已经实现了一个数据库，其中我有两个一对多的关系，但似乎房间不允许这样做。是这样吗？

实体是：

@Entity(tableName = "arete_sheet")
data class EAreteSheet(
    @PrimaryKey val id: Long,
    @ColumnInfo(name = "sheet") val form: Sheets,
    @ColumnInfo(name = "version") val version: Int,
)

@Entity(tableName = "arete_sheet_paragraph")
data class EAreteSheetParagraph(
    @PrimaryKey val id: Long,
    @ColumnInfo(name = "arete_sheet_id") val sheet: Long,
    @ColumnInfo(name = "name") val name: String
)

@Entity(tableName = "arete_sheet_form")
data class EAreteSheetForm(
    @PrimaryKey val id: Long,
    @ColumnInfo(name = "arete_sheet_paragraph_id") val paragraph: Long,
    @ColumnInfo(name = "fieldType") val fieldType: FieldType,
    @ColumnInfo(name = "cell") val cell: String,
    @ColumnInfo(name = "label") val label: String
)

为了解决架构问题，我实现了这些连接类：

data class EAreteSheetWithParagraph(
    @Embedded val sheet: EAreteSheet,
    @Relation(
        parentColumn = "id",
        entityColumn = "arete_sheet_id"
    )
    val paragraph: List<EAreteSheetParagraphWithForm>
)
data class EAreteSheetParagraphWithForm(
    @Embedded val paragraph: EAreteSheetParagraph,
    @Relation(
        parentColumn = "id",
        entityColumn = "arete_sheet_paragraph_id"
    )
    val forms: List<EAreteSheetForm>
)

这是 DAO 实现：

  @Transaction
    @Query("SELECT * FROM arete_sheet")
    suspend fun getSheetWithParagraphsAndForms(): List<EAreteSheetWithParagraph>

这是他在构建阶段给我的错误：

app/build/generated/source/kapt/debug/it/ximplia/agri2000/model/db/dao/AreteSheetDAO_Impl.java:203: error: constructor EAreteSheetWithParagraph in class EAreteSheetWithParagraph cannot be applied to given types;
              _item = new EAreteSheetWithParagraph();
                      ^
  required: EAreteSheet,List<EAreteSheetParagraphWithForm>
  found: no arguments
  reason: actual and formal argument lists differ in length
app/build/generated/source/kapt/debug/it/ximplia/agri2000/model/db/dao/AreteSheetDAO_Impl.java:204: error: sheet has private access in EAreteSheetWithParagraph
              _item.sheet = _tmpSheet;

我认为 Room 不允许级联解决依赖关系，但我想知道是否有人成功，或者我在更改代码之前是否犯了错误。

【问题讨论】：

标签： android sql kotlin android-room

【解决方案1】：

在 EAreteSheetWithParagraph 中，您指定了 EAreteSheetParagraphWithForm 的列表

根据：-

val paragraph: List<EAreteSheetParagraphWithForm>

@Relation 将尝试根据 EAreteSheetParagraphWithForm 表单类确定列，该类不是实体。您应该更改 @Relation 以使用 entity 参数指定适当的实体（即 EAreteSheetParagraph）。

所以 EAreteSheetWithParagraph 应该是这样的：-

data class EAreteSheetWithParagraph(
    @Embedded val sheet: EAreteSheet,
    @Relation(
        entity = EAreteSheetParagraph::class,
        parentColumn = "id",
        entityColumn = "arete_sheet_id" 
    )
    val paragraph: List<EAreteSheetParagraphWithForm>
)

但是，我不相信您已经走到了这一步，因为这些消息似乎在抱怨具有表格类型的 sheet 变量（并且该变量是私有的）。

如果不添加 entity= 参数，那么编译，如果达到那个阶段，将会失败：-

> Task :app:kaptDebugKotlin FAILED
error: The class must be either @Entity or @DatabaseView. - a.a.so68953488kotlinroommany1_n.EAreteSheetParagraphWithForm
E:\AndroidStudioApps\SO68953488KotlinRoomMany1N\app\build\tmp\kapt3\stubs\debug\a\a\so68953488kotlinroommany1_n\EAreteSheetWithParagraph.java:12: error: Cannot find the child entity column `arete_sheet_id` in a.a.so68953488kotlinroommany1_n.EAreteSheetParagraphWithForm. Options: 
    private final java.util.List<a.a.so68953488kotlinroommany1_n.EAreteSheetParagraphWithForm> paragraph = null;
                                                                                               ^
The class must be either @Entity or @DatabaseView. - a.a.so68953488kotlinroommany1_n.EAreteSheetParagraphWithForm

我认为 Room 不允许在级联中解决依赖关系，但我想知道是否有人成功，或者我在更改代码之前是否犯了错误。

如上所述，我认为问题出在错误上：-

概念证明

根据您的代码，以下显示嵌套/多个 1-n 确实有效：-

使用您的代码但进行以下更改（以避免问题并简化）：-

@Entity(tableName = "arete_sheet_form")
data class EAreteSheetForm(
    @PrimaryKey val id: Long,
    @ColumnInfo(name = "arete_sheet_paragraph_id") val paragraph: Long,
    @ColumnInfo(name = "fieldType") val fieldType: /* FieldType */ String, // changed for convenience/brevity
    @ColumnInfo(name = "cell") val cell: String,
    @ColumnInfo(name = "label") val label: String
)

fieldType 的类型从 FieldType 更改为 String，因此无需额外的类和类型转换器。

和

@Entity(tableName = "arete_sheet")
data class EAreteSheet(
    @PrimaryKey val id: Long,
    @ColumnInfo(name = "sheet") val form: /*Sheets*/ String, // changed for convenience/brevity
    @ColumnInfo(name = "version") val version: Int,
)

用字符串替换的表格类型

EAreteSheetWithParagraph 同上

EAreteSheetParagraphWithForm 改为

data class EAreteSheetParagraphWithForm(
    @Embedded val paragraph: EAreteSheetParagraph,
    @Relation(
        parentColumn = "id",
        entityColumn = "arete_sheet_paragraph_id",
        entity = EAreteSheetForm::class
    )
    val forms: List<EAreteSheetForm>
)

即根据我的偏好添加了实体参数，以便始终对实体参数进行编码。

使用AreteSheetDao :-

@Dao
abstract class AreteSheetDAO {

    @Insert
    abstract fun insert(eAreteSheet: EAreteSheet): Long
    @Insert
    abstract fun insert(eAreteSheetParagraph: EAreteSheetParagraph): Long
    @Insert
    abstract fun insert(eAreteSheetForm: EAreteSheetForm): Long

    @Transaction
    @Query("SELECT * FROM arete_sheet")
    abstract fun getSheetWithParagraphsAndForms(): List<EAreteSheetWithParagraph>

}

活动中的代码：-

class MainActivity : AppCompatActivity() {

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

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

        var s1 = dao.insert(EAreteSheet(10,"Sheet1",1))
        var s2 = dao.insert(EAreteSheet(20,"Sheet2",1))
        var p1 = dao.insert(EAreteSheetParagraph(100,s1,"Para1 (Sheet1)"))
        var p2 = dao.insert(EAreteSheetParagraph(101,s1,"Para2 (Sheet1)"))
        var p3 = dao.insert(EAreteSheetParagraph(201,s2,"Para3 (Sheet2)"))
        var p4 = dao.insert(EAreteSheetParagraph(202,s2,"Para4 (Sheet2)"))
        var f1 = dao.insert(EAreteSheetForm(1000,p1,"typex","cellx","Form1"))
        var f2 = dao.insert(EAreteSheetForm(1001,p1,"typex","cellx","Form2"))
        var f3 = dao.insert(EAreteSheetForm(1002,p1,"typex","cellx","Form3"))
        var f4 = dao.insert(EAreteSheetForm(1010,p2,"typex","cellx","Form4"))
        var f5 = dao.insert(EAreteSheetForm(1011,p2,"typex","cellx","Form5"))
        var f6 = dao.insert(EAreteSheetForm(1020,p3,"typex","cellx","Form6"))
        val TAG = "ARETEINFO"
        for(sw: EAreteSheetWithParagraph in dao.getSheetWithParagraphsAndForms()) {
            Log.d(TAG,"Sheet ID is ${sw.sheet.id} Form is ${sw.sheet.form} Version is ${sw.sheet.version}" )
            for(pf: EAreteSheetParagraphWithForm in sw.paragraph) {
                Log.d(TAG,"\tPara is ${pf.paragraph.name} etc")
                for(f: EAreteSheetForm in pf.forms) {
                    Log.d(TAG,"\t\tForm is ${f.label}")
                }
            }
        }
    }
}

可以看出，加载了一些数据，2张Sheets，每张有2个段落，然后4个表格分布不均匀（3到para1，2到para2，1到para3，0到para4）。

然后提取数据（作为EAreteSheetWithParagraph的数组），遍历数组（遍历段落中的底层数组和表格）并输出到日志结果是：-

D/ARETEINFO: Sheet ID is 10 Form is Sheet1 Version is 1
D/ARETEINFO:    Para is Para1 (Sheet1) etc
D/ARETEINFO:        Form is Form1
D/ARETEINFO:        Form is Form2
D/ARETEINFO:        Form is Form3
D/ARETEINFO:    Para is Para2 (Sheet1) etc
D/ARETEINFO:        Form is Form4
D/ARETEINFO:        Form is Form5
D/ARETEINFO: Sheet ID is 20 Form is Sheet2 Version is 1
D/ARETEINFO:    Para is Para3 (Sheet2) etc
D/ARETEINFO:        Form is Form6
D/ARETEINFO:    Para is Para4 (Sheet2) etc

【讨论】：