c# - Fluent NHibernate, working with interfaces

Question

I just switched to Fluent NHibernate and I've encountered an issue and did not find any information about it.

Here's the case :

public class Field : DomainObject, IField
{
    public Field()
    {  
    }

    public virtual string Name { get; set; }
    public virtual string ContactPerson { get; set; }
    public virtual bool Private { get; set; }
    public virtual IAddress Address { get; set; }  
}

IAddress is an interface implemented by a class named Address

public class Address : DomainObject, IAddress
{
    public Address()
    {
    }

    public virtual string City { get; set; }
    public virtual string Country { get; set; }
    public virtual string PostalCode { get; set; }
    public virtual string StreetAddress { get; set; }
}

Here's my mapping files for both classes

ADDRESS

public class AddressMap : ClassMap<Address>
{   
    public AddressMap()
    {
        WithTable("Addresses");
        Id(x => x.Id, "Id").Access.AsCamelCaseField(Prefix.Underscore).GeneratedBy.Guid();
        Map(x => x.City, "City");
        Map(x => x.Country, "Country");
        Map(x => x.PostalCode, "PostalCode");
        Map(x => x.StreetAddress, "StreetAddress");
    }
}

FIELD

public class FieldMap : ClassMap<Field>
{
    public FieldMap()
    {
        WithTable("Fields");
        Id(x => x.Id, "Id").Access.AsCamelCaseField(Prefix.Underscore).GeneratedBy.Guid();
        Map(x => x.Name, "Name");
        Map(x => x.ContactPerson, "ContactPerson");
        Map(x => x.Private, "Private");
        References(x => x.Address, "AddressId").Cascade.Delete().Cascade.SaveUpdate();
    }
}

So when I tried to retrive a field object from my database, I get an NHibernate error that states that IAddress is not mapped. Is there any way to specify to NHibernate to use the Address class in the mapping?

Please let me know if more information are needed.

Thanks a lot,

Charles

Answer 1

I find that there are valid reasons for using an interface instead of a concrete class as a property.

For example, if your Field class was in a seperate project to the Address class, and you didn't have a dependency on the Address class's project from the Field class's project.

There are other ways of dealing with this situation, but the simplest way is often to attempt what you are doing and explicity tell NHibernate the concrete class to use for IAddress.

You can now do this in Fluent NHibernate, like this:

References(x => x.Address, "AddressId")
    .Class(typeof(Address);

Unfortunately you can't do this with HasMany or HasManyToMany. I'm not sure if this would even be possible due to lack of good covariance support in C#.