如何在 Room Android 的外键中使用 set_default

Question

I don't understand how use set_default in a foreign key in onDelete.

Where do I set a default value for child row? Please tell me, what can I do?

Answer 1

They are added at the variable declaration in the Entity annotated class.

For example (in Kotlin):

@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"

) { /*...*/ }

The value set for the variables will act as default values.

For extra reference on SET_DEFAULT see this other SO Question with it's accepted answer: Room database: NOT NULL constraint failed at deletion