Fluent NHibernate - many to many with additional table - Cannot insert the value NULL Exception during deletion - nhibernate

Question

I have a many-to-many relationship with additional table and when I am trying to delete a A object which also has a reference in AB table below error occur:
$exception {"could not delete collection: [A.AB#20][SQL:
UPDATE AB SET AId =
null WHERE AId =
#p0]"} NHibernate.Exceptions.GenericADOException
Cannot insert the value NULL into column 'AId',
table 'AB'; column
does not allow nulls. UPDATE fails. The statement has been terminated.
My database schema:
My classes:
public class A
{
public virtual int AId { get; protected set; }
public virtual IList<AB> AB { get; set; }
}
public class B
{
public virtual int BId { get; protected set; }
public virtual IList<AB> AB { get; set; }
}
public class AB
{
public virtual int ABId { get; protected set; }
public virtual A A { get; set; }
public virtual B B { get; set; }
public virtual int CustomProperty { get; set; }
}
My mappings:
public class AMap : ClassMap<A>
{
public AMap()
{
Table("A");
SchemaAction.None();
Id(x => x.AId)
.GeneratedBy.Identity();
HasMany(x => x.AB)
.KeyColumn("AId")
.Cascade.All();
}
}
public class BMap : ClassMap<B>
{
public BMap()
{
Table("B");
SchemaAction.None();
Id(x => x.BId)
.GeneratedBy.Identity();
HasMany(x => x.AB)
.KeyColumn("BId")
.Cascade.All();
}
}
public class ABMap : ClassMap<AB>
{
public ABMap()
{
Table("AB");
SchemaAction.None();
Id(x => x.ABId)
.GeneratedBy.Identity();
Map(x => x.CustomProperty)
.Not.Nullable();
References(x => x.A)
.Column("AId");
References(x => x.B)
.Column("BId")
.Cascade.None();
}
}
Code:
_session.BeginTransaction();
var a = _session.Get<A>(1);
foreach (var ab in a.AB) {
_session.Delete(ab);
}
_session.Delete(a);
transaction.Commit();
I want to delete the record in A table and every associated record in AB table.
The simplest solution is to make AId and BId in AB table nullable but I think there is a better solution and it can be resolved in mappings.
Thank you in advance :)

Answer 1

Try this:
HasMany(x => x.AB)
.KeyColumn("BId")
.Cascade.All()
.Inverse();