title: How to use a choiceField declared in the model, in a form. django
body: <p>I have this in my <code>model.py</code></p>
  
  <pre><code>class marca(models.Model):
      marcas = (
          ('chevrolet', 'Chevrolet'),
          ('mazda', 'Mazda'),
          ('nissan', 'Nissan'),
          ('toyota', 'Toyota'),
          ('mitsubishi', 'Mitsubishi'),
      )
  
      marca = models.CharField(max_length=2, choices= marcas)
      def __unicode__(self):
          return self.marca
  </code></pre>
  
  <p>And I need to use it in my <code>form.py</code>
  I tried this but it doesn't work.</p>
  
  <pre><code>class addVehiculoForm(forms.Form):
      placa                   = forms.CharField(widget = forms.TextInput())
      tipo                    = forms.CharField(max_length=2, widget=forms.Select(choices= tipos_vehiculo))
      marca                   = forms.CharField(max_length=2, widget=forms.Select(choices= marcas))
  </code></pre>