The beginning of the stack trace I got today looked as below:

2009-05-19 11:01:45,078 ERROR [5232] XXX - Speaker already deleted: Castle.ActiveRecord.Framework.ActiveRecordException: Could not perform Delete for Speaker ---> NHibernate.Exceptions.GenericADOException: could not delete collection: [XXX#XXX][SQL: SQL not available] ---> System.Data.SqlClient.SqlException: Cannot insert the value NULL into column 'SpeakerId', table 'XXX.dbo.lnk_Session_Speaker'; column does not allow nulls. UPDATE fails.
The statement has been terminated.
   ...
   at SpeakerProxy3e9fd3b0e82745c2b91b8a353acaa93d.DeleteAndFlush()

Background

I had three tables and classes representing them: Session / app_Session, Speaker / app_Speaker, and SessionSpeaker / lnk_Session_Speaker.

Speaker declared collection of links session-speaker as follows:

[HasMany(Lazy = true, Table = "lnk_Session_Speaker", ColumnKey = "SpeakerId", Cascade = ManyRelationCascadeEnum.Delete)]
public virtual IList SessionSpeakers
{
   get { return _sessions; }
   set { _sessions = value; }
}

The above described problem occured when I tried to delete a speaker who was assigned to a session – there was a row in lnk_Session_Speaker which represented this connection. Speakers who were not linked could be deleted just fine.

Solution

The reason for the difficulties was lack of Inverse = true declaration for collection of links session-speaker in Speaker class. This statement means that this side (from Speaker’s perspective) is the inverse side – that a Speaker is pointed by SessionSpeaker.

[HasMany(Lazy = true, Table = "lnk_Session_Speaker", ColumnKey = "SpeakerId", Cascade = ManyRelationCascadeEnum.Delete, Inverse = true)]
public virtual IList SessionSpeakers
{
   get { return _sessions; }
   set { _sessions = value; }
}

With this link objects (rows from lnk_Session_Speaker table) could be removed first and and then the speaker themselves could be removed.