title: How to customize form django with fields of the diferents Models
body: <p>I need to display a field on a form and another field on a Model in html page.</p>
  
  <pre><code>class CatracaForm(forms.Form):
      ra = forms.CharField(label='RA', max_length=30)
  
  
  class MotivoForm(forms.ModelForm):
      class Meta:
          model = Motivo
          fields = ['descrmotivo', 'obs']
  </code></pre>
  
  <p>In my HTML page I want show ra and descrmotivo field.
  The descrmotivo field  should be a combobox component.</p>
  
  <p>How to make this?</p>