how to get instance of a model form and assign it to its key in view?

Question

During form processing, I'd like to be able to set a foreign key field on a model object from its base model when the user selects a value from the dropdown list.

models.py

class Question(models.Model):
    question = models.CharField(max_length=200)

class Teacher(models.Model):
        name = models.CharField(max_length=200)
        l_name = models.CharField(max_length=200)
    
class Student_Answer(models.Model):
    teacher = models.ForeignKey(Teacher, on_delete=models.CASCADE)   
    question = models.ForeignKey(Question, on_delete=models.CASCADE)
    answer = models.SmallIntegerField(choices=RATING_CHOICES)

For example, I have five records in model A, two records in model B and I have two model forms.

forms.py

class AnswerForm(ModelForm):
    class Meta:
        model = Student_Answer
        fields = ('answer',)
        widgets = { 'answer': RadioSelect(choices=RATING_CHOICES),}

class TeacherForm(ModelForm):
    name = ModelChoiceField(queryset=Teacher.objects.all())
    class Meta:
        model = Teacher
        fields = ('name',)

For the questions, I just assign them directly within the view and then giving the instances to the foreign key after validation. Now, if I do the same for Teacher I mean this teacher = Teacher.objects.get(id=2) and then choice.teacher = teacher it's going to work perfectly. But that is not the case which I want. The teacher will be selected by the user. I am looking for a way like the below view.

views.py

def index(request):
question1 = Question.objects.get(id=6)
question2 = Question.objects.get(id=7)
if request.method == "POST":
    teacher_form = TeacherForm(request.POST)
    form = AnswerForm(request.POST, prefix='question1')
    form1 = AnswerForm(request.POST, prefix='question2')

    if (form.is_valid):
        choice = form.save(commit=False)
        choice.question = question1
        choice.teacher = teacher_form
        choice.save()

        choice = form1.save(commit=False)
        choice.teacher = teacher_form
        choice.question = question2
        choice.save()

        return HttpResponseRedirect('/submited/')

else:
    teacher_form = TeacherForm()
    form = AnswerForm(prefix='question1')
    form1 = AnswerForm(prefix='question2')

context = {
    'teacher_form': teacher_form,
    'form': form,
    'form1': form1,
    'all_questions': Question.objects.all(),
}
return render(request, 'index.html', context)

The problem with the above example is this error:

ValueError: Cannot assign "<TeacherForm bound=True, valid=Unknown, fields=(name)>": "Student_Answer.teacher" must be a "Teacher" instance.

If I include instance choice.teacher = teacher_form.instance then this error will come up:

Exception Value:    
save() prohibited to prevent data loss due to unsaved related object 'teacher'.

And if I save the form in the first line after validation it is going to create a new record for the Teacher class. I do not want to save a new record, I just want to store the selected value in the template and store that value for the Student_Answer class. Is there a way to fix this?

index.html

<form action="{% url 'evaluation:index'%}" method="POST">
  {% csrf_token %}
  {{ teacher_form }}
  {{ all_questions.0 }}
  {{ form }}
  {{ all_questions.1}}
  {{ form1 }}
  
  <input type="submit" value="Vote">
</form>

Gin Fuyou · Accepted Answer · 2021-01-06 09:34:55Z

First of all - a critical problem of your view. You are using several forms (which is not that rare and quite possible) but you validate only single one - but you must validate ALL of them. There was a nice article on doing that, if I find it I'll link it in updates. Your second error comes out exactly because of it - you trying to set instance from a form which is not validated.

 if all([form.is_valid(), form1.is_valid(), teacher_form.is_valid()]):

Second, yes, you don't want to save() that teacher form, you even don't need it to be a ModelForm, just forms.Form with ModelChoiceField will do. You just need to validate it and then access it's cleaned_data

# I'm renaming field to 'teacher' since 'name' is poor choice for Teacher object
teacher = teacher_form.cleaned_data['teacher']

And at last for a number of Question forms, you probably want a model formset (2-3) constant forms may do for current approach but more or varying number of questions will make it really bothersome.

formsets: https://docs.djangoproject.com/en/3.1/topics/forms/modelforms/#model-formsets

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