Fluent NHibernate: Mapping HasManyToMany by convention

Question

I'm using Fluent NHibernate's AutoMap feature to map my entities. Most of my entities inherit from a base class Entity which has a property public IList<Tag> Tags.

The tags are in a separate table in the database, so I use a many-to-many relation. But Fluent NHibernate creates mappings for a one-to-many relation.

I'd like to write a convention to override these mappings to use HasManyToMany(...) if the class inherits from Entity. Is this possible and how?

The convention could either rely on the property's type or its name.

Some code for illustration:

// entities
public class Entity
{
    public virtual int Id { get; set; }
    // ... some other properties
    public virtual IList<Tag> { get; set; }
}

public class Tag
{
    public virtual int Id { get; set; }
    public virtual string TagName { get; set; }
}

public class Event : Entity
{
    // ... some properties
}

// Fluent NHibernate configuration
public static ISessionFactory CreateSessionFactory()
{
    var config = new CustomAutomappingConfiguration();
    return Fluently.Configure()
        .Database(MsSqlConfiguration.MsSql2008.ConnectionString(c => c.FromConnectionStringWithKey("Sql")))
        .Mappings(m =>
        {
            m.AutoMappings.Add(AutoMap.AssemblyOf<Event>(config)
                .IgnoreBase<Entity>()
                .Conventions.Add<CustomForeignKeyConvention>()
                .Conventions.Add<CustomManyToManyTableNameConvention>();
        })
        .BuildSessionFactory();
}

which inheritance strategy do you use? table-per-concrete-class, table-per-classhirarchy or table-per-subclass? — Firo
I'm not sure, which one this is exactly. Maybe an example can explain: I have Event : Entity which maps to the table named 'Event' containing all properties of Event and the base class Entity. — davehauser
this would be table-per-concrete-class. There i only know of Override<> for each subclass, which is not what you wanted or heavy use of reflection to call override with the dynamically filtered types — Firo
Override<> (or implementations of IAutoMappingOverride<>) for each entity is exactly, what I could find so far :-) It works, but I have to remember, when I add another entity. Unfortunately an override of the mapping for Entity has no effect... Thanks anyway! — davehauser

codeprogression codeprogression · Accepted Answer · 2011-07-26T18:14:38

I don't think you can accomplish the mapping with conventions. However, if you want to keep one linking table between the entities and tags, you can do the following:

m.AutoMappings.Add(AutoMap.AssemblyOf<Event>(config)
    .IncludeBase<Entity>()    
    .Override<Entity>(map => 
         map.HasManyToMany(e => e.Tags)
            .Inverse()
            .Cascade.SaveUpdate()));

Notice that I changed IgnoreBase<Entity>() to IncludeBase<Entity>(). This will add an Entity table, but will keep one linking table. With this mapping, you will get the following table DDL:

create table [Entity] (
    Id INT IDENTITY NOT NULL,
   primary key (Id)
)

create table TagToEntity (
    Entity_id INT not null,
   Tag_id INT not null
)

create table Event (
    Entity_id INT not null,
   primary key (Entity_id)
)

create table [Tag] (
    Id INT IDENTITY NOT NULL,
   TagName NVARCHAR(255) null,
   primary key (Id)
)

alter table TagToEntity 
    add constraint FKD7554554A8C4CA9 
    foreign key (Tag_id) 
    references [Tag]

alter table TagToEntity 
    add constraint FKD75545564C9EC79 
    foreign key (Entity_id) 
    references [Entity]

alter table Event 
    add constraint FKA2FD7DF664C9EC79 
    foreign key (Entity_id) 
    references [Entity]

If you choose to do an Override<> per subclass, you will have a linking table per subclass.

Fluent NHibernate: Mapping HasManyToMany by convention

2 Answers