Formset documentation should include a note about form.blah_ptr for derived models

[Andriy Drozdyuk]

Whenever a model inherits from a non-abstract model, it shares it's primary key with it.
As a consequence, the form produced by inlineformset_factory no longer has id field, by the "parentname_ptr" field.

For example imagine you have a case such as:

class Car(models.Model):
    color = #string field

class BMW(Car):
    shop = models.ForeignKey(CarShop)

Then in our view:

car = BMW.objects.get(pk=1)
BMWFromset = inlineformset_factory(CarShop, BMW, 
                                can_delete=True,
				extra=1)
formset = BMWFormset(instance=car)

The in our html:

<form method="post">
{{formset.management_form}}
<ul>
	{% for form in formset.forms %}
	
        <li>
        <!-- NOTICE that we do NOT use form.id here -->
        {{form.car_ptr}}
        {{form}}
        </li>
	
        {% endfor %}
</ul>
<input type="submit"  value="submit" />
</form>

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Tim Graham]

Also reported in #20113

[Mariusz Felisiak]

​PR

[Mariusz Felisiak <felisiak.mariusz@…>]

In 0b7378db:

Fixed #11097 -- Added note about parent link fields in formsets for multi-table inheritance models.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 56061de:

[3.0.x] Fixed #11097 -- Added note about parent link fields in formsets for multi-table inheritance models.

Backport of 0b7378db1fdd7bfc7b78089811b3abb722c4ba95 from master

[Mariusz Felisiak <felisiak.mariusz@…>]

In 8c1b492e:

[2.2.x] Fixed #11097 -- Added note about parent link fields in formsets for multi-table inheritance models.

Backport of 0b7378db1fdd7bfc7b78089811b3abb722c4ba95 from master