Django: Prepopulate extra fields in a Formset with a ManyToMany Model

Question

I have two models with a ManyToMany (M2M) relationship. In order to have extra fields on the M2M Model, I defined a through= model. I tried setting up a inlineformset_factory, using parent=ChildModel and model=ParentModel.parent_child.through as described in this thread pendant to inline formsets for many-to-many relations. But it did not solve my issue.

How can I access (i.e. prepopulate) and update the extra fields of the M2M model, when using formsets?

models.py

class ParentModel(models.Model):
    name = model.CharField()
    parent_child = model.ManyToManyField(ChildModel, through='ParentChildModel')


class ChildModel(models.Model):
    name = model.CharField()
    

class ParentChildModel(models.Model):
    parent = models.ForeignKey(ParentModel, on_delete=models.CASCADE)
    child = models.ForeignKey(ChildModel, on_delete=models.CASCADE)
    extra_field_1 = models.CharField()

forms.py

class ChildForm(forms.ModelForm):

    class Meta:
        model = ChildModel
        fields = '__all__'



ChildFormset = modelformset_factory(ChildModel,
                                     form=ChildForm,
                                     extra=0,
                                     can_delete=True,
                                     can_order=True,
                                     )

T_Torture · Accepted Answer · 2022-07-18T05:57:58.253

I found a solution, setting up a ModelFormset on the ChildModel. For the extra field of the ParentChildModel, I defined a field in form.py which I manually populate using the code down below.

views.py

# get instances
parent_instance = Parent.objects.get(id=pk)
parent_children = parent_instance.parent_child.all()
parent_child_instance = ParentChildModel.objects.filter(parent=parent_instance)

# initializing & prepopulate forms
parent_form = ParentForm(request.POST or None, instance=parent_instance)
child_formset = ChildFormset(request.POST or None, queryset=parent_children)
for child_form in child_formset:
    child_form.fields['extra_field_1'].initial = parent_child_instance.get(child=child_form.instance).extra_field_1

forms.py

class ChildForm(forms.ModelForm):
    extra_field_1 = forms.CharField()

    class Meta:
        model = ChildModel
        fields = '__all__'


ChildFormset = modelformset_factory(ChildModel,
                                    form=ChildForm,
                                    ...
                                    )

score -1 · Answer 2 · answered Jul 15 '22 at 13:28

-1

You would need to create a formset with the ParentChildModel and an instance of ChildModel instead of just using the ChildModel as it won't be able to access extra_field_1.

The documentation suggests an inline-formset structure like this:

from django.forms import inlineformset_factory

ParentChildFormset = inlineformset_factory(ChildModel, ParentChildModel, ...)
child = ChildModel.objects.get(...)
formset = ParentChildFormset(instance=child)

https://docs.djangoproject.com/en/4.0/topics/forms/modelforms/#inline-formsets

answered Jul 15 '22 at 13:28

0sVoid

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@OsVoid If you pass in a ChildModel instance in the Formset, that you are missing the FK of the ParentModel to get the right instances in the ParentChildModel. My logic would suggest that I need a Formset based on the ParentChildModel and pass in the respective instances. But like this, I struggle with getting the Data from the ParentModel and ChildModel. – T_Torture Jul 17 '22 at 08:39

Django: Prepopulate extra fields in a Formset with a ManyToMany Model

2 Answers2