Delete without giving error

Question

I have two models, one has the foreign key in the other.

I wanted to implement a delete, that would prevent her from deleting that key when she was already safe in a relationship.

In the view.py I implemented, but I can’t delete any. I think the error is in if that’s in view.

Models py.

class Horario(models.Model):
    horario = models.CharField('Horário', max_length=11, help_text='Ex= 00:00/00:00')

class Alocar(models.Model):
    data = models.DateField('Data', auto_now=True, blank=True)
    horario = models.ForeignKey(Horario, on_delete=models.PROTECT)
    turma = models.ForeignKey(Turma, on_delete=models.PROTECT)
    sala = models.ForeignKey(Sala, on_delete=models.PROTECT)

view py.

@login_required
def DelHorario(request, id):
    context = {}
    horario = get_object_or_404(Horario, pk=id)

    if request.method == 'POST':
        if horario.id > 0:
            messages.success(request, 'Não pode ser excluído, pois está sendo usado numa relação')
        else:
            horario.delete()
        return redirect('alocar:addhorario')
    context['horario'] = horario
    return render(request, 'alocar/delhorario.html')

Answer 1

Live the problem is that you cannot effectively delete a record if it exists in another table like fk, it violates referential integrity. Being in Django admin you don’t need to do anything it already validates this and shows a message. However, in custom code you can validate by checking if in the relational table there are records with the operator "select_related".

py views.

@login_required
def DelHorario(request, id):
    values = Alocar.objects.select_related('horario').filter(horario__id=id)
    if request.method == 'POST':
        if values:
            messages.success(request, 'Não pode ser excluído, pois está sendo usado numa relação')
        else:
            horario = get_object_or_404(Horario, pk=id)
            horario.delete()
        return redirect('alocar:addhorario')
    return render(request, 'alocar/delhorario.html')