NHibernate, adding children via parent vs setting reference to parent on children

Question

I'm currently working on my first NHibernate project.

To test it out I am building an entity graph and that graph I try to persist to the database. When I add children to the parent via the add on the IList the insert is not working in the database because I get null column exceptions on the database (code snippet 1)

1. When I set a reference on the child to the parent it is working (though this feels not natural for me, see code snippet
2. Is this normal behavior or am I doing something wrong?

Snippet 1:

var country = new Country();
var countryLocale = new CountryLocale { LangCode = "nl-nl", Name = "Nederland" };
country.CountryLocales.Add(countryLocale);

var city = new City();
var cityLocale = new CityLocale { LangCode = "nl-nl", Name = "Amsterdam" };
city.CityLocales.Add(cityLocale);
country.Cities.Add(city);

Snippet 1 Error:

Cannot insert the value NULL into column 'CountryId', table 'ArtWorld.dbo.City'; column does not allow nulls. INSERT fails.\r\nThe statement has been terminated.

snippet 2:

var country = new Country();

var countryLocale = new CountryLocale { Country = country,LangCode = "nl-nl", 
                                        Name = "Nederland" };
var city = new City{Country = country};

var cityLocale = new CityLocale { City = city, LangCode = "nl-nl", 
                                        Name = "Amsterdam" };

session.SaveOrUpdate(country);

City and CityLocale Map:

public class CityMap : ClassMap<City>
{
    public CityMap()
    {
        Table("City");
        Id(x => x.Id);
        References(x => x.Country).Column("CountryId").Cascade.SaveUpdate();
        HasMany(x => x.CityLocales).KeyColumn("CityId").Cascade.SaveUpdate();
    }
}

public class CityLocaleMap : ClassMap<CityLocale>
{
    public CityLocaleMap()
    {
        Table("City_Locale");
        Id(x => x.Id);
        Map(x => x.LangCode).Not.Nullable();
        Map(x => x.Name).Not.Nullable();
        References(x => x.City).Column("CityId").Cascade.SaveUpdate();
    }
}

this is my Country/Countrylocale map:

public class CountryMap : ClassMap<Country>
{
    public CountryMap()
    {
        Table("Country");
        Id(x => x.Id);
        HasMany(x => x.Cities).KeyColumn("CountryId").Cascade.SaveUpdate();
        HasMany(x => x.CountryLocales).KeyColumn("CountryId").Cascade.SaveUpdate();
    }

}

public class CountryLocaleMap :ClassMap<CountryLocale>
{
    public CountryLocaleMap()
    {
        Table("Country_Locale");
        Id(x => x.Id);
        Map(x => x.LangCode).Not.Nullable();
        Map(x => x.Name).Not.Nullable();
        References(x => x.Country).Column("CountryId").Cascade.SaveUpdate();
    }
}

Answer 1

NHibernate Mapping changes :

On the Country side you have

  HasMany(x => x.CountryLocales).KeyColumn("CountryId").Cascade.SaveUpdate()

On the CountryLocale side you have

References(x => x.Country).Column("CountryId").Cascade.SaveUpdate();

One of the relations should be marked as inverse using .Inverse().

The .Cascase.SaveUpdate() makes sure that NHibernate manages the life cycle of the child entity CountryLocale when a transient child is added to the parent Country's list of CountryLocales. If you dont want to explicitly handle the life cycle of CountryLocale yourself , I would suggest to mark the Many-To-One side from CountryLocale -> Country as Inverse.

There are two ways you could make your referencing Country in CountryLocale less of a sour eye and more intutive

If Country manages the Locales

Add a method in Country which will manage the Locales which are being added. A client (user of Country and CountryLocale need not explicitly play with the references)

public virtual bool AddCountryLocales(CountryLocale locale)
{
        if(!this.CountryLocales.Contains(locale))
        {
              locale.Country =this;
              this.CountryLocales.Add(locale);
              return true;  
        }
        return false;
}

CountryLocale is more domain driven

By redefining it such that you cannot have a CountryLocale without a Country

public class CountryLocale
{
    public CountryLocale(Country country)
    {
         this.Country = country;
    }

    //you need a no-agrument constructor for NHibernate
    protected CountryLocale()
    {
    }
}