Modify one form instance in a Django formset? - django

Question

I have a Django formset in which I render four instances of a form. Each form in the formset has two fields, but I want the last (4th) instance to only show/input one field. How can I do this, or is there a better way? I tried limiting the formset to three fields and making the fourth instance its own form, but I need to validate that field1 against the field1 fields in the formset. I couldn't see how to validate a form against a simultaneously submitted formset.
views.py:
FormSet = formset_factory(MyForm, formset=BaseMyFormSet, extra=4)
.html:
<form action="" method="post">{% csrf_token %}
{{ formset.management_form }}
{{ formset.non_form_errors }}
<div>
{% for form in formset %}
{% for field in form %}
{{ field.errors }}
{{ field.label_tag }}: {{ field }}
{% endfor %}
{% endfor %}
</div>
<p><input type="submit" value="Submit" /></p>
</form>
The formset has two fields in forms.py:
class MyForm(forms.Form):
# field1
# field2
How do I make it so the first three forms in the formset have two fields, but the last only contains field1?

Answer 1

Try explicitly removing the field for the last form in your views.py:
def view_func(request):
FormSet = formset_factory(MyForm, formset=BaseMyFormSet, extra=4)
if request.method == 'POST':
formset = FormSet(request.POST, request.FILES)
# assuming this is a required field for the other forms
formset.forms[-1].fields['field2'].required = False
if formset.is_valid():
...
else:
formset = FormSet()
del formset.forms[-1].fields['field2']
return render(request, 'form.html', {'formset': formset})
Any further adjustments depend on your form and validation logic. BaseMyFormSet.clean() is the appropriate place for formset-wide validation and it sounds like you already have code there to compare field1 across forms.
(From a purist sense, it might be better to have the FormSet class handle this entirely, but this is easier. FormSet code is decently complex. It'd make sense to override BaseFormSet.forms() but with that #cached_property bit, you're involved with implementation details best left to Django.)