title: Django 1.9 form inheritance on UpdateView
body: <p>The problem i have is with a single field that i added. </p>
  
  <p>I have my form:</p>
  
  <pre><code>
  
      class employeesForm(forms.ModelForm):
          class Meta:
              fields = [
                  'num_employee',
                  'id_ignition',
                  'fullName',
                  'shortName',
                  'salary',
                  'gender',
                  'rfc',
                  'imss',
                  'curp',
                  'birthday',
                  'initday',
                  'id_costCenter',
                  'id_jobCode',
                  'id_status',
                  'nationality',
                  'picture',
                  'source',
              ]
  
  
              widgets = {
                  'num_employee': forms.NumberInput(attrs={'class': 'form-control', 'name': 'num_employee'}),
                  'id_ignition': forms.NumberInput(attrs={'class': 'form-control', 'name': 'id_ignition'}),
                  'fullName': forms.TextInput(attrs={'class': 'form-control', 'name': 'fullName', 'placeholder': 'Angel Rafael Ortega Vazquez'}),
                  'shortName': forms.TextInput(attrs={'class': 'form-control', 'name': 'shortName', 'placeholder': 'Rafael Ortega'}),
                  'salary': forms.NumberInput(attrs={'class': 'form-control', 'name': 'salary', 'placeholder': '5000'}),
                  'gender': forms.CheckboxInput(attrs={'class': 'form-control', 'name': 'gender'}),
                  'rfc': forms.TextInput(attrs={'class': 'form-control', 'name': 'rfc', 'id': 'rfc'}),
                  'imss': forms.TextInput(attrs={'class': 'form-control', 'name': 'imss', 'id': 'imss'}),
                  'curp': forms.TextInput(attrs={'class': 'form-control', 'name': 'curp'}),
                  'birthday': forms.DateInput(attrs={'class': 'form-control', 'name': 'birthday'}),
                  'initday': forms.DateInput(attrs={'class': 'form-control', 'name': 'initday'}),
                  'id_costCenter': forms.Select(attrs={'class': 'form-control', 'name': 'id_costCenter'}),
                  'id_jobCode': forms.Select(attrs={'class': 'form-control', 'name': 'id_jobCode'}),
                  'id_status': forms.Select(attrs={'class': 'form-control', 'name': 'id_status'}),
                  'nationality': forms.Select(attrs={'class': 'form-control', 'name': 'nationality'}),
                  'picture': forms.ClearableFileInput(attrs={'class': 'form-control', 'name': 'picture'}),
                  'source': forms.Select(attrs={'class': 'form-control', 'name': 'source'}),
              }
  
  </code></pre>
  
  <p>In my model I have an extra field (antiquity), but I’m ignoring it here because in my CreateView i populate that field based in other parameters (antiquity = initday).</p>
  
  <p>Everything worked fine until I needed to create an UpdateView with that extra field pre-populated, what I did was to inherit the previews form and adding that extra field:</p>
  
  <pre><code>
  
      class Enhanced_employeesForm(employeesForm):
  
          antiquity = forms.CharField(
              widget=forms.DateInput(attrs={'class': 'form-control', 'name': 'antiquity'}))
  
  </code></pre>
  
  <p>It did the thing, the input field is rendered in my template, but while everything is pre-populated with the info based in the id, my antiquity field is empty.</p>
  
  <p>That’s the only thing I’m missing in my configuration, because django even detect when that field is empty on the submit and prevent any update to the database</p>
  
  <p>I tried doing something like:</p>
  
  <pre><code>form.antiquity(instance=Employees.objects.get(pk=self.kwargs['pk']))
  </code></pre>
  
  <p>But the error says that 'Enhanced_employeesForm' object has no attribute 'antiquity'.</p>