title: get id of object saved in django forms.Form
body: <p>models.py:</p>
  
  <pre><code>class Invoice(models.Model):
      invoice_issued_on = models.DateField()
  </code></pre>
  
  <p>forms.py:</p>
  
  <pre><code>class InvoiceForm(forms.Form):
      invoice_issued_on = forms.DateField(
          input_formats=['%d/%m/%Y'],
          initial=today.strftime("%d/%m/%Y"),
          label=mark_safe('Issued on'),
          widget=forms.TextInput(attrs={ 'placeholder': 'DD/MM/YYYY', 'data-date-format':'DD/MM/YYYY', 'class': 'form-control text-center' })
      )
  
      def save(self):
          data = self.cleaned_data
          invoice=Invoice(invoice_issued_on=data['invoice_issued_on'])
          instance=invoice.save()
  </code></pre>
  
  <p>Question:
  How do i get the id (database id) of the Invoice object saved in the database? Warning i am not using forms.ModelForm so instance.id doesn't work here. For technical reason i cannot use ModelForm.</p>