Fluent NHibernate generates extra columns

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

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 ListOfFirstClass { get; set; }
}

public class FirstClassMap : ClassMap
{
    public FirstClassMap()
    {
        References(x => x.MyReference);
    }
}

public class SecondClassMap : ClassMap
{
    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
{
    public SecondClasssMap()
    {
        HasMany(x => x.ListOfFirstClass).KeyColumn("MyReference_id");
    }
}

or:

public class FirstClassMap : ClassMap
{
    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.