In unit tests, back reference is not set for one-to-one association

package myapp

class House {
  String name

  static hasOne = [address: Address]

  static constraints = {
  }
}

package myapp

class Address {
  String street

  House house

  static constraints = {
  }
}

package myapp

import grails.test.mixin.TestMixin
import grails.test.mixin.domain.DomainClassUnitTestMixin
import org.junit.Test

@TestMixin(DomainClassUnitTestMixin)
class HouseUnitTests {
  @Test
  public void testBackReferenceUnit() {
    mockDomain(Address)
    mockDomain(House)

    def address = new Address(street:"Street 001")
    def house = new House(name:"Some house", address: address)

    house.save(flush:true)

    assert house.id != null
    assert address.house != null
  }
}