如何使用Xodus DNQ级联删除最后一个链接的项目

Question

我有两个实体； municipality和city（实际上更多，但此处简化）。 municipality必须始终至少包含一个城市。

是否有一种设置约束的方法，该约束使得最终的city级联删除也可以删除其父级municipality？

实体

class XdCity(entity: Entity) : XdEntity(entity) {
    companion object : XdNaturalEntityType<XdCity>()

    var name by xdRequiredStringProp()
    var municipality: XdMunicipality by xdLink1(
        XdMunicipality::cities,
        onDelete = OnDeletePolicy.CLEAR,
        onTargetDelete = OnDeletePolicy.CASCADE
    )
}

class XdMunicipality(entity: Entity) : XdEntity(entity) {
    companion object : XdNaturalEntityType<XdMunicipality>()

    var name by xdRequiredStringProp(unique = true)
    val cities by xdLink1_N(
        XdCity::municipality,
        onDelete = OnDeletePolicy.CASCADE,
        onTargetDelete = OnDeletePolicy.CLEAR
    )
}

测试用例

@Test
fun testCityDeletionCascade() {
    Database.store.transactional {
        val municipality = XdMunicipality.findOrNew("Mun 1")
        XdCity.findOrNew("City A").apply {
            this.municipality = municipality
        }
        XdCity.findOrNew("City B").apply {
            this.municipality = municipality
        }
    }

    Database.store.transactional { 
        XdCity.all().first().delete()
        assertTrue { XdMunicipality.all().isNotEmpty }

        XdCity.all().first().delete()            
        assertTrue { XdMunicipality.all().isEmpty }
    }
}