android - 如何在 Room Android 的外键中使用 set_default

Question

我不明白如何在 onDelete 的外键中使用 set_default。

我在哪里设置子行的默认值？请告诉我，我能做什么？

Answer 1

它们被添加到 Entity 注解类的变量声明处。

例如(在 Kotlin 中):

@Entity
data class MyClass (

    var id: Long = 0L

    // index is recommended for foreign keys to avoid full table scans on 'child column's table')
    @ColumnInfo (name = "name", index = true)
    var name_ChildRow: String = "default-name"

    // Explicit defaultValue in column info, excluding this could give Not Null constraint fail exceptions
    // in onDeletes and onUpdates set_default but shouldn't actually be needed.
    // (In case room generates a query which states something other; this default value
    // in columnInfo annotation should enforce the default value).
    @ColumnInfo (name = "parent_id", defaultValue = "1", index = true)
    var yetAnotherTableParentId: Long = "1L"

    @ColumnInfo (name = "string_null")
    var stringNull: String = "Null"

) { /*...*/ }

为变量设置的值将作为默认值。

有关 SET_DEFAULT 的额外引用，请参阅其他 SO 问题及其已接受的答案:Room database: NOT NULL constraint failed at deletion