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在我的 Kotlin Android 代码库中，我有以下 2 个实体..

@Entity(
tableName = ModuleConfiguration.tableName,
primaryKeys = [ModuleConfiguration.COL_ID],
foreignKeys = arrayOf(
    ForeignKey(
        entity = Module::class,
        parentColumns = [Module.COL_ID],
        childColumns = [ModuleConfiguration.COL_MODULE_ID],
        onDelete = ForeignKey.CASCADE
    ),
    ForeignKey(
        entity = Group::class,
        parentColumns = [Group.COL_ID],
        childColumns = [ModuleConfiguration.COL_GROUP_ID]
    )
)
)
class ModuleConfiguration(
@ColumnInfo(name = COL_ID)
var id: String = UUID.randomUUID().toString(),

@ColumnInfo(name = COL_TABLE)
var table: String,

@ColumnInfo(name = COL_FIELD_NAME)
var fieldName: String,

@ColumnInfo(name = COL_FIELD_LABEL)
var fieldLabel: String,

@ColumnInfo(name = COL_FIELD_TYPE)
var fieldType: ModuleConfigurationItemType,

@ColumnInfo(name = COL_GROUP_ID)
var groupId: String?,

@ColumnInfo(name = COL_MODULE_ID)
var moduleId: String,

@ColumnInfo(name = COL_POSITION)
var position: Int,

@ColumnInfo(name = COL_VISIBLE)
var visible: Int = 1, //Usually visible

@ColumnInfo(name = COL_READ_ONLY)
var readOnly: Int = 0, //Usually false

@ColumnInfo(name = COL_REQUIRED)
var required: Int = 0, //Usually false

@ColumnInfo(name = COL_CREATED_AT)
var createdAt: Long = CustomDateTimeUtil.getTodayInUTC(),

@ColumnInfo(name = COL_UPDATED_AT)
var updatedAt: Long = CustomDateTimeUtil.getTodayInUTC()
) : Cloneable, Serializable, Parcelable  {

constructor(parcel: Parcel) : this(
    parcel.readString() ?: "",
    parcel.readString() ?: "",
    parcel.readString() ?: "",
    parcel.readString() ?: "",
    fieldType = ModuleConfigurationItemType.valueOf(parcel.readString() ?: FieldType.UNKNOWN.name),
    groupId = parcel.readString(),
    moduleId = parcel.readString() ?: "",
    position = parcel.readInt(),
    visible = parcel.readInt(),
    readOnly = parcel.readInt(),
    required = parcel.readInt(),
    createdAt = parcel.readLong(),
    updatedAt = parcel.readLong()
) {
}

fun getViewType() : ModuleConfigurationItemType {
    return this.fieldType
}

override fun equals(other: Any?): Boolean {
    return super.equals(other)
}

override fun hashCode(): Int {
    return super.hashCode()
}

override fun clone(): Any {
    return super.clone()
}

override fun writeToParcel(parcel: Parcel, flags: Int) {
    parcel.writeString(id)
    parcel.writeString(table)
    parcel.writeString(fieldName)
    parcel.writeString(fieldLabel)
    parcel.writeString(fieldType.name)
    parcel.writeString(groupId)
    parcel.writeString(moduleId)
    parcel.writeInt(position)
    parcel.writeInt(visible)
    parcel.writeInt(readOnly)
    parcel.writeInt(required)
    parcel.writeLong(createdAt)
    parcel.writeLong(updatedAt)
}

override fun describeContents(): Int {
    return 0
}

companion object CREATOR : Parcelable.Creator<ModuleConfiguration> {

    const val tableName = "module_configuration"

    const val COL_ID = "id"
    const val COL_MODULE_ID = "module_id"
    const val COL_TABLE = "table"
    const val COL_FIELD_NAME = "field_name"
    const val COL_FIELD_LABEL = "field_label"
    const val COL_FIELD_TYPE = "field_type"
    const val COL_GROUP_ID = "group_id"
    const val COL_VISIBLE = "visible"
    const val COL_READ_ONLY = "read_only"
    const val COL_REQUIRED = "required"
    const val COL_POSITION = "position"
    const val COL_CREATED_AT = "created_at"
    const val COL_UPDATED_AT = "updated_at"
    const val COL_CLIENT_ID = "client_id"

    override fun createFromParcel(parcel: Parcel): ModuleConfiguration {
        return ModuleConfiguration(parcel)
    }

    override fun newArray(size: Int): Array<ModuleConfiguration?> {
        return arrayOfNulls(size)
    }
}
}

和集团实体

@Entity(
tableName = Group.tableName,
primaryKeys = [Group.COL_ID]
)
class Group(
@ColumnInfo(name = COL_ID)
var id: String = UUID.randomUUID().toString(),

@ColumnInfo(name = COL_NAME)
var name: String,

@ColumnInfo(name = COL_CREATED_AT)
var createdAt: Long = CustomDateTimeUtil.getTodayInUTC(),

@ColumnInfo(name = COL_UPDATED_AT)
var updatedAt: Long = CustomDateTimeUtil.getTodayInUTC()
) : Cloneable, Parcelable  {

constructor(parcel: Parcel) : this(
    parcel.readString() ?: "",
    parcel.readString() ?: "",
    parcel.readLong(),
    parcel.readLong()
) {
}

override fun equals(other: Any?): Boolean {
    return super.equals(other)
}

override fun hashCode(): Int {
    return super.hashCode()
}

override fun clone(): Any {
    return super.clone()
}

override fun writeToParcel(parcel: Parcel, flags: Int) {
    parcel.writeString(id)
    parcel.writeString(name)
    parcel.writeLong(createdAt)
    parcel.writeLong(updatedAt)
}

override fun describeContents(): Int {
    return 0
}

companion object CREATOR : Parcelable.Creator<Group> {

    const val tableName = "group"

    const val COL_ID = "id"
    const val COL_NAME = "name"
    const val COL_CREATED_AT = "created_at"
    const val COL_UPDATED_AT = "updated_at"
    const val COL_CLIENT_ID = "client_id"

    override fun createFromParcel(parcel: Parcel): Group {
        return Group(parcel)
    }

    override fun newArray(size: Int): Array<Group?> {
        return arrayOfNulls(size)
    }
}
}

我的问题是我正在尝试设置可为空的外键，正如您在名为 ModuleConfiguration 的实体中看到的那样，有一个名为 group_id 的列，它是可为空的。

@ColumnInfo(name = COL_GROUP_ID)
var groupId: String?,

由于此列属于名为 Group 的其他实体，因此我尝试将其设为外键，但是当我这样做时，出现以下错误

SQLiteConstraintException：外键约束失败（代码 787）

在网上搜索时，我在stackoverflow上找到了以下答案：

Android with Room - 如何在房间上设置外键可为空 或 可为空的外键

但这对我没有帮助，因为这些答案建议将原始类型（如 int 或 long）更改为非原始类型（如 Integer 或 Long）以允许它们为空。我已经在使用 String 作为类型。

任何帮助将不胜感激。

提前致谢。

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