python - Accessing parent model instance from modelform of admin inline

Question

I'm using a TabularInline in Django's admin, configured to show one extra blank form.

class MyChildInline(admin.TabularInline):
    model = MyChildModel
    form = MyChildInlineForm
    extra = 1

The model looks like MyParentModel->MyChildModel->MyInlineForm.

I'm using a custom form so I can dynamically lookup values and populate choices in a field. e.g.

class MyChildInlineForm(ModelForm):

    my_choice_field = forms.ChoiceField()

    def __init__(self, *args, **kwargs):
        super(MyChildInlineForm, self).__init__(*args, **kwargs)

        # Lookup ID of parent model.
        parent_id = None
        if "parent_id" in kwargs:
            parent_id = kwargs.pop("parent_id")
        elif self.instance.parent_id:
            parent_id = self.instance.parent_id
        elif self.is_bound:
            parent_id = self.data['%s-parent'% self.prefix]

        if parent_id:
            parent = MyParentModel.objects.get(id=parent_id)
            if rev:
                qs = parent.get_choices()
                self.fields['my_choice_field'].choices = [(r.name,r.value) for r in qs]

This works fine for the inline records bound to an actual record, but for the extra blank form, it doesn't display any values in my choice field, since it doesn't have any record id and there can't lookup the associated MyParentModel record.

I've inspected all the values I could find (args, kwargs, self.data, self.instance, etc) but I can't find any way to access the parent object the tabular inline is bound to. Is there any way to do this?

Answer 1

Update: As of Django 1.9, there is a def get_form_kwargs(self, index) method in the BaseFormSet class. Hence, overriding that passes the data to the form.

This would be the Python 3 / Django 1.9+ version:

class MyFormSet(BaseInlineFormSet):
    def get_form_kwargs(self, index):
        kwargs = super().get_form_kwargs(index)
        kwargs['parent_object'] = self.instance
        return kwargs


class MyForm(forms.ModelForm):
    def __init__(self, *args, parent_object, **kwargs):
        self.parent_object = parent_object
        super(MyForm, self).__init__(*args, **kwargs)


class MyChildInline(admin.TabularInline):
    formset = MyFormSet
    form = MyForm

---

For Django 1.8 and below:

To pass a value of a formset to the individual forms, you'd have to see how they are constructed. An editor/IDE with "jump to definition" really helps here to dive into the ModelAdmin code, and learn about the inlineformset_factory and it's BaseInlineFormSet class.

From there you'll find that the form is constructed in _construct_form() and you can override that to pass extra parameters. It will likely look something like this:

class MyFormSet(BaseInlineFormSet):
    def _construct_form(self, i, **kwargs):
        kwargs['parent_object'] = self.instance
        return super(MyFormSet, self)._construct_form(i, **kwargs)

    @property
    def empty_form(self):
        form = self.form(
            auto_id=self.auto_id,
            prefix=self.add_prefix('__prefix__'),
            empty_permitted=True,
            parent_object=self.instance,
        )
        self.add_fields(form, None)
        return form

class MyForm(forms.ModelForm):
    def __init__(self, *args, **kwargs):
        self.parent_object = kwargs.pop('parent_object', None)
        super(MyForm, self).__init__(*args, **kwargs)


class MyChildInline(admin.TabularInline):
    formset = MyFormSet
    form = MyForm

Yes, this involves a private _construct_form function.

update Note: This doesn't cover the empty_form, hence your form code needs to accept the parameters optionally.