C# – NHibernate : object references an unsaved transient instance

When I try to add an object to a repository I get the NHibernate.TransientObjectException with message:

object references an unsaved transient instance – save the transient instance before flushing or set cascade action for the property to something that would make it autosave.
Type: MyApp.Domain.Model.Task

I use:

• NHibernate 3.3.1
• FluentNHibernate 1.4
• Microsoft Unity 2.1.505

The method in the repository is more or less as follows (I present here a merged version):

public void Add(T item)
{
    if (!GetSession().Query<T>().Contains(item)) //generates the error, if commented out works fine
    {
        try
        {
            GetSession().BeginTransaction();
            GetSession().Save(item);
            GetSession().CommitTransaction();
        }    
        catch
        {
            //rollback code
        }
    }
}

The important thing is that the object I try to save in the repository does not have any references and/or relations to other objects. It is a rather simple entity.
What is worth mentioning that it extends the DomainObject which has Guid type ID.

I use the Code First schema, so my DB is generated based on my code.
Because of inheritance (every entity extends DomainObject) I use UseUnionSubclassForInheritanceMapping()

The whole stack trace is as follows:

   w NHibernate.Engine.ForeignKeys.GetEntityIdentifierIfNotUnsaved(String entityName, Object entity, ISessionImplementor session)
   w NHibernate.Type.EntityType.GetIdentifier(Object value, ISessionImplementor session)
   w NHibernate.Type.ManyToOneType.NullSafeSet(IDbCommand cmd, Object value, Int32 index, ISessionImplementor session)
   w NHibernate.Param.NamedParameterSpecification.Bind(IDbCommand command, IList`1 multiSqlQueryParametersList, Int32 singleSqlParametersOffset, IList`1 sqlQueryParametersList, QueryParameters queryParameters, ISessionImplementor session)
   w NHibernate.Param.NamedParameterSpecification.Bind(IDbCommand command, IList`1 sqlQueryParametersList, QueryParameters queryParameters, ISessionImplementor session)
   w NHibernate.SqlCommand.SqlCommandImpl.Bind(IDbCommand command, ISessionImplementor session)
   w NHibernate.Loader.Loader.PrepareQueryCommand(QueryParameters queryParameters, Boolean scroll, ISessionImplementor session)
   w NHibernate.Loader.Loader.DoQuery(ISessionImplementor session, QueryParameters queryParameters, Boolean returnProxies)
   w NHibernate.Loader.Loader.DoQueryAndInitializeNonLazyCollections(ISessionImplementor session, QueryParameters queryParameters, Boolean returnProxies)
   w NHibernate.Loader.Loader.DoList(ISessionImplementor session, QueryParameters queryParameters)
   w NHibernate.Loader.Loader.ListIgnoreQueryCache(ISessionImplementor session, QueryParameters queryParameters)
   w NHibernate.Loader.Loader.List(ISessionImplementor session, QueryParameters queryParameters, ISet`1 querySpaces, IType[] resultTypes)
   w NHibernate.Hql.Ast.ANTLR.Loader.QueryLoader.List(ISessionImplementor session, QueryParameters queryParameters)
   w NHibernate.Hql.Ast.ANTLR.QueryTranslatorImpl.List(ISessionImplementor session, QueryParameters queryParameters)
   w NHibernate.Engine.Query.HQLQueryPlan.PerformList(QueryParameters queryParameters, ISessionImplementor session, IList results)
   w NHibernate.Impl.SessionImpl.List(IQueryExpression queryExpression, QueryParameters queryParameters, IList results)
   w NHibernate.Impl.AbstractSessionImpl.List(IQueryExpression queryExpression, QueryParameters parameters)
   w NHibernate.Impl.ExpressionQueryImpl.List()
   w NHibernate.Linq.DefaultQueryProvider.ExecuteQuery(NhLinqExpression nhLinqExpression, IQuery query, NhLinqExpression nhQuery)
   w NHibernate.Linq.DefaultQueryProvider.Execute(Expression expression)
   w NHibernate.Linq.DefaultQueryProvider.Execute[TResult](Expression expression)
   w System.Linq.Queryable.Contains[TSource](IQueryable`1 source, TSource item)
   w MyApp.Data.Nhibernate.Fluent.Repository`1.Add(T item) w f:\Projects\MyApp\MyApp.Data.Nhibernate.Fluent\Repository.cs:wiersz 75
   w MyApp.Domain.Services.Impl.CatalogServices.CreateTask(Task task) w f:\Projects\MyApp\MyApp.Domain.Services\Impl\CatalogServices.cs:wiersz 51

I would appreciate your help

Best Regards,
Sebastian