Django | <title> object (None)>" needs to have a value for field "id" before this many-to-many relationship can be used

Question

I get an error using with using form.save(commit=False) and I cannot figure out what I'm doing wrong.

My view looks something like this:

views.py

def make_electable(request, election_id):
    election = get_object_or_404(CommissionElection, id=election_id)
    if request.method == 'POST':
        form = MakeElectable(request.POST)
        if form.is_valid():
            electable = form.save(commit=False)
            electable.election.set(election)
            form.save()

forms.py

class MakeElectable(forms.ModelForm):
    class Meta:
          model = ElectablePerson
          fields = ['commission',]
    commission = forms.ModelChoiceField(queryset=Commissie.objects.all(), widget=forms.RadioSelect())

models.py

class CommissionElection(models.Model):
    title = models.CharField(max_length=64)
    commission = models.ManyToManyField(Commissie)
    electable_time = DateRangeField()
    voting_time = DateRangeField()


class ElectablePerson(models.Model):
    commission = models.CharField(max_length=128)
    election = models.ManyToManyField(CommissionElection)

The error I get is:

"<ElectablePerson: ElectablePerson object (None)>" needs to have a value for field "id" before this many-to-many relationship can be used.

I don't see why the object is None. I suspect it has something to do with the manytomanyfield.

Answer 1

A many to many field is a table which uses 'id_electable, id_election' as keys, with commit=false, you don't persist which gives you no id. Following code should fix your issue.

if form.is_valid():
    electable = form.save()
    electable.election.add(election)

Also see https://docs.djangoproject.com/en/dev/ref/models/relations/#django.db.models.fields.related.RelatedManager.add for many to many relations.