title: Django: render inherited (nested) values as a list in forms.ModelForm
body: <p>I'm trying to display a dropdown list in my form, from my model  'TipoDocumento' (I need to display the column 'nombre_corto' as a list).</p>
  
  <p>1) My model name is "Cocinera", Cocinera inherits from my model "Usuario".<br>
  2) "Usuario" inherits it's 'documento' field from my model "Documento".<br>
  3) "Documento" model inherits it's 'tipo_documento' field from "TipoDocumento".     </p>
  
  <p>But I cannot render 'tipo_documento', as a list, from my 'Cocinera' model through my 'CocineraForm' form. I get an error, detailed at the end.</p>
  
  <p><strong><em>All my models are in 'nucleo' app. 
  Only the form that gets rendered is in my other app 'app_administrador'.</em></strong></p>
  
  <p><strong>================================</strong>    </p>
  
  <p><strong>Nucleo - The app is just named nucleo</strong></p>
  
  <p><strong>================================</strong>   </p>
  
  <p><strong>MODELS:</strong></p>
  
  <p><strong><em>Model 'TipoDocumento'</em></strong></p>
  
  <pre><code>from django.db import models
  
  class TipoDocumento(models.Model):
          nombre_corto = models.CharField(blank=False, null=False, max_length=25)
          nombre_largo = models.CharField(blank=False, null=False, max_length=100)
  
      def __str__(self):
          return self.nombre_corto
  </code></pre>
  
  <p><strong><em>Model 'Documento'</em></strong></p>
  
  <pre><code>class Documento(models.Model):
  
      def __str__(self):
          return self.codigo
  
      tipo_documento = models.ForeignKey(TipoDocumento, on_delete=models.SET_NULL, null=True)
      codigo = models.CharField(max_length=25)
  </code></pre>
  
  <p><strong><em>Model 'Usuario':</em></strong></p>
  
  <pre><code>class Usuario(models.Model):
      class Meta:
          abstract = True
  
      nombre = models.CharField(blank=False, null=False, max_length=200)
      apellido_paterno = models.CharField(blank=False, null=False, max_length=100)
      apellido_materno = models.CharField(blank=True, null=False, max_length=100)
      fecha_de_nacimiento = models.DateField(blank=False, null=False)
      documento = models.OneToOneField(Documento, on_delete=models.CASCADE, blank=False, null=True)
  </code></pre>
  
  <p><strong><em>Model 'Cocinera':</em></strong></p>
  
  <pre><code>class Cocinera(Usuario):
  
      habilidades = models.ForeignKey(Habilidad, blank=True, null=True)
      experiencia_con_mascotas = models.BooleanField(default=False)
  
  
      def __str__(self):
          return self.nombre
  </code></pre>
  
  <p><strong>================================</strong>    </p>
  
  <p><strong>app_administrador</strong></p>
  
  <p><strong>================================</strong>  </p>
  
  <p><strong><em>My FORM</em></strong></p>
  
  <pre><code>class CocineraForm(forms.ModelForm):
      class Meta:
          model = Cocinera
          fields = ['nombre', 'apellido_paterno', 'apellido_materno', 'tipo_documento', 'documento', 'fecha_de_nacimiento', 'direccion', 'telefono_o_celular' , 'latitud_y_longitud',
                    'genero', 'antecedentes_policiales', 'foto']
          widgets = {
              'fecha_de_nacimiento': DateInput()
          }
  </code></pre>
  
  <p>Related question: 
  According to this <a href="https://stackoverflow.com/questions/14901300/use-django-modelchoice-field-to-create-pulldown-to-lookup-table?rq=1">Use Django ModelChoice field to create pulldown to lookup table?</a></p>
  
  <p>I've added:</p>
  
  <pre><code>tipo_documento = forms.ModelChoiceField(queryset=TipoDocumento.objects.all(),
                                               empty_label=None)
  </code></pre>
  
  <p>But still when I run my app, I get: </p>
  
  <pre><code>File "/home/ogonzales/Escritorio/web_envs/
  cocineras_env/lib/python3.5/site-packages/django/forms/models.py",
  line 277, in __new__raise FieldError(message)
  django.core.exceptions.FieldError: Unknown field(s) 
  (tipo_documento) specified for Cocinera
  </code></pre>