// All the objects in the collection already exist in the database
// so NHibernate should throw an Exception because of a PK constraint
// violation
foreach (var existingEntity in existingEntities)
{
    // Implementation calls Session.CreateTransaction(),
    // but the interface returns System.Data.IDbTransaction.
    // This assembly has no references to NHibernate.
    using (var transaction = repo.CreateTransaction())
    {
        try
        {
            // Executes a simple insert
            repo.SaveComposite(existingEntity);
            transaction.Commit();
        }
        catch (Exception e)
        {
            transaction.Rollback();

            // Only record the inner-most message
            var innerException = e;
            while (innerException.InnerException != null)
                innerException = innerException.InnerException;

            Errors.Add(new Error(ErrorCode.ExceptionOccurred, innerException.Message));
        }
    }
}