private void OnCathedraDeleted(object sender, ModifiedEntityEventArgs<Cathedra> e)
 {
     var cathedra = e.ModifiedEntity;
     Cathedras.RemoveSingle(c => c.Model.Equals(cathedra));
 }
        private void OnCathedraCreated(object sender, ModifiedEntityEventArgs<Cathedra> e)
        {
            var cathedra = e.ModifiedEntity;
            var cathedraViewModel = CathedraTreeItemViewModel.FromModel(
                model: cathedra,
                owner: this,
                professorRepository: _professorRepository);

            Cathedras.Add(cathedraViewModel);
        }