nhibernate mapping by code approach, mapping object

Question

I have following model

public class Car : Entity<int>
{
    public virtual int Id{ get; set; }
    ...
    public virtual Engine Engine { get; set; }

}

and I'm using nhibernate mapping by code approach

public class CarMap : ClassMapping<Car>
{
    public CarMap()
    {
        Id(x => CarId, m => m.Generator(Generators.Identity));
        // how map reference Engine?  
        **// edit**
        HasOne(x=>x.Engine, m=>{}) // is this good enough?

    }
}

how map Engine in this CarMap object?

Fran · Accepted Answer · 2017-02-10 15:19:55Z

You need a little more information in the question, but here's a couple of options.

Is this really a one to one relationship? One to one relationships are somewhat unique in that both sides of the relationship tend to share the same Id. Like David Osborne said you most likely want a One to Many relationship. But you you want it to be bi-directional? i.e. you can navigate down from the Engine to all the cars that may have that engine or up from the car to a specific engine. i.e. engine is Chrysler Hemi engine 5.7L and it is in the cars, Ram Pickup, Dodge Durango, Dodge Charger.

Then you may want to map the objects like this

public class Engine : Entity<int>
{
    public Engine()
    {
        Cars = new List<Car>();
    }

    public virtual int Id { get; protected set; }
    public virtual decimal Displacement { get; set; }
    //more properties

    public virtual IList<Car> Cars { get; }

    public virtual void AddCar(Car car)
    {
        if (Cars.Contains(car)) return;

        Cars.Add(car);
    }

    public virtual void RemoveCar(Car car)
    {
        if (!Cars.Contains(car)) return;

        Cars.Remove(car);
    }
}

public class Car : Entity<int>
{
    public virtual int Id { get; set; }
    public virtual Engine Engine { get; set; }

}

So if you are mapping the Engine you need to define the Cars mapping list this

        Bag(x => x.Cars, map =>
        {
            map.Key(k => k.Column(col => col.Name("EngineId")));
            map.Cascade(Cascade.All | Cascade.DeleteOrphans); //optional
        },
            action => action.OneToMany());

and the other side of the relationship like this

        ManyToOne(x => x.Engine, map =>
        {
            map.Column("EngineId");
            map.NotNullable(true); // if you require the Engine to be in a car
        });

If you just want a one way mapping from the car to the engine, just remove all the references to the Cars list and delete the Bag mapping.

David Osborne · Accepted Answer · 2017-02-10 11:39:28Z

0

Using Fluent NH, I would use References(), which has a mapping-by-code equivalent of ManyToOne() apparently.

answered Feb 10, 2017 at 11:39

David Osborne

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2 Comments

user1765862 Over a year ago

Are you sure it's ManyToOne not OneToOne?

David Osborne Over a year ago

I haven't used mapping-by-code, I'm just going on what the linked article says. It's from 2012, so things might have changed since...

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