Loop on boundfields at init of inline sub-forms breaks the form set

[Jonathan Rossel]

When doing a loop (even a dummy one) on their boundfields at the init phase, the sub-forms of an inline form set are "broken", i.e. the parent key field is rendered instead of being hidden and many additional SQL requests are generated.

In this illustration case, I have 2 related models - Fig and Line - where each Fig can have many Line(s).

forms.py

class FigForm( ModelForm ):
    class Meta:
        model = Fig
        fields = '__all__'

class LineForm( ModelForm ):
    class Meta:
        model = Line
        fields = '__all__'
    
    def __init__(self, *args, **kwargs):
        super( LineForm, self ).__init__(*args, **kwargs)
    
        for bf in self:   # <- this screws everything
            pass          #    even with a dummy loop
    
LineFormSet = inlineformset_factory( Fig, Line, form=LineForm, extra=1 ) 

In real life, I use this loop in a custom model form class to add default CSS classes to the form widgets, but the additionnal SQL requests kill my server.

---

Additional information

model.py

class Fig(models.Model):
    name = models.CharField(max_length=200,unique=True)
    xlabel = models.CharField(max_length=200,blank=True)
    ylabel = models.CharField(max_length=200,blank=True)
    descr = models.TextField(blank=True)
    pub_date = models.DateTimeField('date published',auto_now=True)
    
    def __str__(self):
        return self.name
        
    def get_absolute_url(self):
        return reverse('webplot:detail', kwargs={'pk':self.pk} )
        
    
class Line(models.Model):
    fig = models.ForeignKey(Fig, on_delete=models.CASCADE)
    x = models.CharField(max_length=200)
    y = models.CharField(max_length=200)
    label = models.CharField(max_length=20)
    
    def __str__(self):
        return self.label + ': y(x)=' + self.y + ' for x=' + self.x
        

views.py

def detail(request,pk='new'):
    # Used for both CREATE and UPDATE
    # create a form instance and populate it with data from the request if any
    if pk == 'new':
        fig = Fig() # instanciate empty object
    else:
        fig = get_object_or_404( Fig, pk=pk )
    
    form = FigForm( request.POST or None, instance=fig )
    subform = LineFormSet( request.POST or None, instance=fig )
    
    if request.method == 'POST':
        if form.is_valid() and subform.is_valid():
            form.save()
            subform.save()
            return redirect(fig)
    
    ctx = {'form': form, 'subform': subform }
       
    return render(request, 'webplot/detail.html', ctx )

detail.html

      <form method="post" class="w3-container">{% csrf_token %}
            
            <h4>Figure settings</h4>
            <table class="w3-table">
                {{ form.as_table }}
            </table>
            
            <h4>Line settings</h4>
            {{ subform.management_form }}
            {{ subform.non_field_errors }}
            <table class="w3-table">
                {% for thisform in subform.forms %}
                    {% if forloop.first %}
                        <thead>
                        <tr class="w3-border-bottom">
                        {% for field in thisform.visible_fields %}
                            <th>{{ field.label_tag }}</th>
                        {% endfor %}
                        </tr>
                        </thead>
                        <tbody>
                    {% endif %}
                    <tr>
                        {{ thisform.non_field_errors }}
                        {% for field in thisform.hidden_fields %}
                            {{ field }}
                        {% endfor %}
                        {% for field in thisform.visible_fields %}
                            <td>{{ field.errors }}{{ field }}</td>
                        {% endfor %}
                    </tr>
                {% endfor %}
                </tbody>
            </table>
            <input type="submit" value="Submit" class="w3-btn" />
        </form>

[Tim Graham]

I don't think I've every seen a need to iterate over bound fields in __init__() -- certainly that's not a documented pattern. Can you instead make your modifications using self.fields['field_name'].widget in Form.__init__()?

[Jonathan Rossel]

Replying to Tim Graham:

I don't think I've every seen a need to iterate over bound fields in __init__() -- certainly that's not a documented pattern. Can you instead make your modifications using self.fields['field_name'].widget in Form.__init__()?

Hi,

Thanks for the hint! It does work nicely when accessing .widget from fields['field_name'] instead of passing by the bound field. I guess it must come from a difference in the modification location in the initialization sequence (?). After re-reading the docs, I still don't grasp the exact difference between fields and bound fields though, and why what I've done should go against a documented pattern. The only things I've found are:

• "If you want to make one widget instance look different from another, you will need to specify additional attributes at the time when the widget object is instantiated and assigned to a form field (and perhaps add some rules to your CSS files)." ​https://docs.djangoproject.com/en/1.11/ref/forms/widgets/#styling-widget-instances. That does not say "but before the bound field is instanciated"
• "If you want to provide some additional classes in addition to the error and required classes that may be required, you can provide those classes as an argument (of the BoundField instance css_classes method)." ​https://docs.djangoproject.com/en/1.11/ref/forms/api/#methods-of-boundfield. That was probably what led me to this loop on bound fields...

At this point, I am not sure if this ticket should stay as a bug or should be converted to a request for documentation improvement (possibly combined with a modification of the code to raise an exception if someone does what I did).

Anyway, thanks for the help !

[Tim Graham]

I opened #28655 to add some documentation about your use case. I don't think trying to prevent iterating bound fields is worthwhile and I can't think of a simple way to do that, offhand.