modelformset_factory does not honor extra parameter - django

Question

Django: 1.4.1
Model:
class Hoja(models.Model):
nombre = models.CharField(max_length=200) # requerido
class Linea(models.Model):
hoja = models.ForeignKey(Hoja) # requerido
nombre = models.CharField(max_length=200) # requerido
padre = models.ForeignKey('self', null=True, blank=True, related_name='hijo')
View:
lineas = Linea.objects.filter(hoja=alt).order_by('id')
LineaHojaSet = modelformset_factory(Linea, can_delete=True, extra=1 if request.POST.has_key('siguiente') else 0)
formset = LineaHojaSet(request.POST or None, queryset=lineas)
if request.method=='POST':
# process formset
return render_to_response('template.html', {'formset':formset}, context_instance=RequestContext(request))
Template:
<table>
<thead>
<tr><th>Nombre</th><th>Borrar</th></tr>
</thead>
<tbody>
{% for fs in formset %}
<tr>
<td>{{ fs.nombre }}</td>
<td>{{ fs.id }}</td>
</tr>
{% endfor %}
</tbody>
</table>
<input type="submit" name="siguiente" value="Añadir siguiente" />
When I submit the "siguiente" button, I can see than the formset is getting the correct extra field of 1, but in the webpage, the only rows showing are the database ones. It's this a bug, or I'm doing something wrong?

Answer 1

Formset factory finds number of forms either by max_num, extra parameters or form-TOTAL_FORMS parameter in request.POST (or data) from management form.
In your case, request.POST['form-TOTAL_FORMS'] has number which does not include extra form . So it does not add extra form when you create formset.
One solution would be to increment this number by one when your condition is met. e.g.
data = None
if request.POST:
data = request.POST.copy() #required as request.POST is immutable
if request.POST.has_key('siguiente'):
data['form-TOTAL_FORMS'] = int(data['form-TOTAL_FORMS']) + 1
#now use data instead of request.POST
formset = LineaHojaSet(data, queryset=lineas)
....
However, there are some drawbacks of manipulating formset this way. When you validate formset, the extra form will show errors if there are any required fields.
Better solution would be to create formset again before passing it template with one extra form and queryset. Most likely, when formset is valid, you would save any new objects, those will get added by queryset. So your page will show newly added objects and one extra form.
lineas = Linea.objects.filter(hoja=alt).order_by('id')
LineaHojaSet = modelformset_factory(Linea, can_delete=True,)
formset = LineaHojaSet(request.POST or None, queryset=lineas)
if request.method=='POST':
# process formset
if formset.is_valid:
#saved and done with formset.
if request.POST.has_key('siguiente'):
LineaHojaSet = modelformset_factory(Linea, can_delete=True, extra=1)
formset = LineaHojaSet(queryset=lineas)
...
return render_to_response('template.html', {'formset':formset}, context_instance=RequestContext(request))