Fluent NHibernate generates extra columns - nhibernate

Question

We are using Fluent NHibernate for data object model in the company i work.
A couple of days ago, we encountered an issue that Fluent NHibernate generates an extra column which does exist neither in model nor in mapping. Here is the situation:
My Model: FirstClass.cs
public class FirstClass
{
public virtual int Id { get; private set; }
public virtual SecondClass MyReference { get; set; }
public virtual DateTime DecisionDate { get; set; }
}
My Mapping:
public class FirstClassMap : ClassMap<FirstClass>
{
public FirstClassMap()
{
Id(x => x.Id);
Map(x => x.DecisionDate);
References(x => x.MyReference);
}
}
After building the schema with the following code,
Instance._sessionFactory = Fluently.Configure()
.Database(MySQLConfiguration.Standard
.ConnectionString(connectionString)
.ShowSql())
.ExposeConfiguration(c =>
{
c.Properties.Add("current_session_context_class", ConfigurationHelper.getSetting("SessionContext"));
})
.ExposeConfiguration(BuildSchema)
.Mappings( m => m.FluentMappings.AddFromAssemblyOf<Community>())
.BuildSessionFactory();
An extra column named "SecondClass_id" is produced with index and foreign key to SecondClass table with Id column. Here is the table produced:
CREATE TABLE `FirstClass` (
`Id` int(11) NOT NULL AUTO_INCREMENT,
`DecisionDate` datetime DEFAULT NULL,
`MyReference_id` int(11) DEFAULT NULL,
`SecondClass_id` int(11) DEFAULT NULL,
PRIMARY KEY (`Id`),
KEY `MyReference_id` (`MyReference_id`),
KEY `SecondClass_id` (`SecondClass_id`),
CONSTRAINT `FK4AFFB59B2540756F` FOREIGN KEY (`MyReference_id`) REFERENCES `SecondClass` (`Id`),
CONSTRAINT `FK4AFFB59B51EFB484` FOREIGN KEY (`SecondClass_id`) REFERENCES `SecondClass` (`Id`),
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
I found that, if I rename "MyReference" to "SecondClass" (same name as the class type), there is no extra column created. But i want to use my property with the name i specified, not with the class name. Why that extra column is created? How do i fix that? I don't want extra foreign key columns hanging around.

Answer 1

This often happens when you're using FNH and you have a two-way relationship between entities.
public class FirstClass
{
public virtual SecondClass MyReference { get; set; }
}
public class SecondClass
{
public virtual List<FirstClass> ListOfFirstClass { get; set; }
}
public class FirstClassMap : ClassMap<FirstClass>
{
public FirstClassMap()
{
References(x => x.MyReference);
}
}
public class SecondClassMap : ClassMap<SecondClass>
{
public SecondClassMap()
{
HasMany(x => x.ListOfFirstClass);
}
}
To fix this you have to override the column name used in either ClassMap, like so:
public class SecondClassMap : ClassMap<SecondClass>
{
public SecondClasssMap()
{
HasMany(x => x.ListOfFirstClass).KeyColumn("MyReference_id");
}
}
or:
public class FirstClassMap : ClassMap<FirstClass>
{
public FirstClassMap()
{
References(x => x.MyReference).Column("SecondClass_id");
}
}
The reason for this is that FNH treats each mapping as a separate relationship, hence different columns, keys, and indexes get created.