How to cascade deletion of last linked item with Xodus DNQ

Question

I have two entities; municipality and city (actually more but simplifying here). A municipality must always contain at least one city.

Is there a way to set up a constraint that makes the deletion of the final city cascade to delete its parent municipality as well?

---

Entities

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 case

@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 }
    }
}

Answer 1

Xodus-dnq has two mechanism for that: XdEntityListener or XdEntity#beforeFlush. Both of them can be applied here. For beforeFlush:

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()
            }
        }
    }

Both of this mechanisms called only on transaction flush. So test should be modified like this:

@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 }
        }
    }