Passing data to formsets.

Using Django
1

How do I pass some initial data to a formset ? What I am trying to do is

  1. Render a formset
  2. After the form is submitted, get the cleaned data and save it to request.session["some_key"]
  3. if request.session["some_key"] is not None, I pass it to the instantiated formset class in the get method of a CBV.

This works fine for other normal forms I have but I face difficulty in formsets. For normal forms, I could just save the data in session and then pass that in GET method of the View to have the form repopulated.

2

You need to instantiate the formset with the initial parameter. Formsets | Django documentation | Django

Did you mean form here?

3

I did. The following is an example snippet.

from django.shortcuts import render
from django.views import View
from .forms import ExampleFormSet

class ExampleView(View):

    def get(self, request, *args, **kwargs):
        data = { 'form-TOTAL_FORMS': '3', 'form-INITIAL_FORMS': '0', }
        example_form = ExampleFormSet(data)
        context = { "example_form": example_form }
        if request.session.get("saved_data") is not None:
            print(request.session.get("saved_data"))
            example_form = ExampleFormSet(initial=request.session.get("saved_data"))
            context = { "example_form": example_form }
            return render(request, "base/main.html", context)
        return render(request, "base/main.html", context)

    def post(self, request, *args, **kwargs):
        data = { 'form-TOTAL_FORMS': '3', 'form-INITIAL_FORMS': '0', }
        example_form = ExampleFormSet(request.POST)
        if example_form.is_valid():
            request.session["saved_data"] = example_form.cleaned_data

        context = { "example_form": example_form }
        return render(request, "base/main.html", context)

forms.py

from django import forms
from django.forms import formset_factory, BaseFormSet

class ExampleForm(forms.Form):

    sample_field = forms.FloatField(
        widget = forms.TextInput(attrs = { "required": "required" })
    )

ExampleFormSet = formset_factory(ExampleForm, extra=3)

This however, causes 3 more fields to appear on every GET request to the ExampleView

4
  1. TOTAL_FORMS and INITIAL_FORMS are variables calculated by Django for validation purposes. You don’t have to pass them when instantiating a formset:
data = { 'form-TOTAL_FORMS': '3', 'form-INITIAL_FORMS': '0', }
example_form = ExampleFormSet(data)

  1. This however, causes 3 more fields to appear on every GET request to the ExampleView

This is the expected behaviour.

ExampleFormSet = formset_factory(ExampleForm, extra=3)

The extra arg indicates that everytime the formset is instantiated, it should include 3 empty forms.

  1. If you need to define a max n° of forms you could use max_forms
ExampleFormSet = formset_factory(ExampleForm, max_forms=3)
5

If I don’t pass these in, I always get the management form error.

6

How are you rendering the formset in the template?

From the docs:

The management form is available as an attribute of the formset itself. When rendering a formset in a template, you can include all the management data by rendering
{{ my_formset.management_form }} (substituting the name of your formset as appropriate).

7

Thanks for pointing this out. It worked !