如何使用Xodus DNQ级联删除最后一个链接项

我有两个实体；市政当局和城市实际上更多，但在这里简化了。一个自治市必须始终包含至少一个城市

有没有一种方法可以设置一个约束，使最终城市级联的删除也可以删除其上级城市

实体

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Xodus dnq有两种机制：XdEntityListener或XdEntitybeforeFlush。它们都可以在这里应用。对于刷新前：

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
    )
}

@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 }
    }
}

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
        )

        override fun beforeFlush() {
            if (cities.isEmpty) {
                delete()
            }
        }
    }

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

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