Deleting foreign keys by fluent Nhibernate

I have a problem inserting data into the table after deleting data. It works the first time or if I restart the app then it works.

public class SingleTestCycle
{
public virtual int Id { get; protected set; }
public virtual int OpeningTime { get; set; }
...
public virtual CompleteOneTest CompleteOneTest { get; set; }
}

public class CompleteOneTest
{
public virtual int Id { get; protected set; }
public virtual string StartTime { get; set; }
public virtual string EndTime { get; set; }
public virtual IList<SingleTestCycle> SingleTest { get; set; }

Mappings are like as follows:

public class SingleTestCycleMap : ClassMap<SingleTestCycle>
{
public SingleTestCycleMap()
{
    Id(x => x.Id);
    Map(x => x.DoorOpeningTime);
    Map(x => x.DoorClosingTime);
    Map(x => x.FlowValveOpeningTime);
    Map(x => x.FlowValveClosingTime);
    References(x => x.CompleteOneTest);
   }
}


public class CompleteOneTestMap : ClassMap<CompleteOneTest>
{
public CompleteOneTestMap()
{
    Id(x => x.Id);
    Map(x => x.StartTime);
    Map(x => x.EndTime);
    HasMany(x => x.SingleTest)
      .Inverse()
      .Cascade.All();
}

}

Trying to delete it in the following way.

I get the following error:

    System.Data.SqlClient.SqlException (0x80131904): The INSERT statement conflicted with the FOREIGN KEY constraint "FK2B1FA9F15AE67907". The conflict occurred in database "MTScada", table "dbo.CompleteOneTest", column 'Id'.
The statement has been terminated.
   at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)

sp_help shows there is PK__Complete__3214EC07E08D05D2 in the management studio.

I tried to delete by the following way but did not help

                    ExecuteSQLQuery("DELETE FROM  SingleTestCycle WHERE [CompleteOneTest_id]=" + "'" + item + "'");
                    Thread.Sleep(200);
                    ExecuteSQLQuery("DELETE FROM  CompleteOneTest WHERE Id=" + "'" + item + "'");