title: I want that Django model field can have only two possible values
body: <pre><code>class Proposta(models.Model):
       descrizione = models.TextField()
       titolo = models.CharField(max_length=200)
       richiedibile = models.BooleanField(default=False)
       inRichiesta = models.BooleanField(default=False)
       archiviata = models.BooleanField(default=False)
  
       # tesi or AP
       tipologia = models.CharField(max_length=50)
  </code></pre>
  
  <p>I want that the 'tipologia' field can have only two possible value: 'tesi' or 'AP'. In other words I want the field looks like a list in which user can  choose the value he want.</p>