Django Form - setting data

I'm improving my English, be patient

My form is a ModelForm and all the necessary data is sent by the user, but I want dynamically set the field ["viagem"] with the last object in the queryset.

How to set a field after sending the data

def cadastro(request):
    dono = Dono.objects.get(user=request.user)
    if request.method == "POST":
        form = VendaForm(dono, request.POST)

        # Here I get the necessary data to call my qs
        colocador_id = form["colocador"].value()
        viagem = Colocador.objects.get(pk=colocador_id).viagem_set.last()
       
        # I want something like this
        form["viagem"] = viagem
        
        if form.is_valid():
            form.save()
        else:
            print('error')
            print(form.errors)
    else:
        form = VendaForm(dono)
    context = {"form": form, }
    return render(request, 'dashboard/cadastro.html', context)

print(form.errors) => <ul class="errorlist"><li>viagem<ul class="errorlist"><li>This field is required</li></ul></li></ul>

Then you should not add this as a Form field. You thus exclude it from the fields in your ModelForm and work with:

from django.contrib.auth.decorators import login_required
from django.shortcuts import get_object_or_404

@login_required
def cadastro(request):
    dono = get_object_or_404(Dono, user=request.user)
    if request.method == 'POST':
        form = VendaForm(dono, request.POST)
        if form.is_valid():
            form.instance.viagem = form.cleaned_data['colocador'].viagem_set.last()
            form.save()
        else:
            print('error')
            print(form.errors)
    else:
        form = VendaForm(dono)
    context = {'form': form, }
    return render(request, 'dashboard/cadastro.html', context)

Note: It is often better to use get_object_or_404(…) [Django-doc], then to use .get(…) [Django-doc] directly. In case the object does not exists, for example because the user altered the URL themselves, the get_object_or_404(…) will result in returning a HTTP 404 Not Found response, whereas using .get(…) will result in a HTTP 500 Server Error.

Note: You can limit views to a view to authenticated users with the @login_required decorator [Django-doc].