title: Which Django model field goes with a CheckboxSelectMultiple form field?
body: <p>I have a django form that allows a user to select multiple options:</p>
  
  <pre><code>CARDS = (
      ("visa", "Visa"),
      ("mastercard", "MasterCard"),
  )
  
  class PaymentForm(forms.ModelForm):
      credit_cards = forms.MultipleChoiceField(choices=CARDS, widget=forms.CheckboxSelectMultiple)
      # ... etc.
  </code></pre>
  
  <p>I have the form's associated model setup as:</p>
  
  <pre><code>class Payment(models.Model):
      user = models.OneToOneField(User)
      credit_cards = models.CharField(choices=CARDS, max_length=100)
      # ... etc.
  </code></pre>
  
  <p>But I'm thinking that a CharField with the choices parameter can only accept a single choice because my form never validates and I get an error like: </p>
  
  <p><strong>Value u"[u'visa']" is not a valid choice.</strong></p>
  
  <p>And it sure looks like a valid choice.</p>
  
  <p>I've seen that <a href="https://stackoverflow.com/questions/2726476/django-multiple-choice-field-checkbox-select-multiple">some people</a> get this working with a ManyToManyField on the model side (which I'd expect) but building a model just for a static list of credit card types seems overkill.</p>
  
  <p>So: is there a specific model field type or different form configuration I should be using to support multiple selections from a pre-defined list of options?</p>
  
  <p>Thanks.</p>