title: How to combine django choice field with filepathfield
body: <p>I want to process teplates with django. For that i need to use Choicefield and FilePathField.</p>
  
  <p>I don't really know how to combine them. The Choice field in the form  works as expected </p>
  
  <pre><code>class TaskBlockForm(forms.ModelForm):
      """ form used for creating a taskblock
          :param forms.ModelForm: superclass of forms.
      """
      users = forms.ModelMultipleChoiceField(queryset=DemoUser.objects.all())
  
      upload_by_file = forms.BooleanField()
      file = forms.FileField(label=_('Init script'), required=False)
  
      template = forms.ChoiceField(widget=forms.Select(), choices=[(0, 'WorldDB'), (1, 'ArticleDB'), (2, 'SampleDB')])
      date = forms.DateField(widget=SelectDateWidget(), label=_('maturity date'), initial=timezone.now())
  </code></pre>
  
  <p>The Model looks like </p>
  
  <pre><code>def generate_templatename(instance, filename):
      """
      helper method to generate path and filename for a template sql file of a taskblock
      :param instance: taskblock instance
      :param filename: filename
      :return: relative url of file path
      """
      url = "template/%s/%s" % (instance.name, filename)
      return url
  
  
  class TaskBlock(models.Model):
      """
          model of a taskblock
      """
      name = models.CharField(max_length=20)
      description = models.TextField()
      create_stamp = models.DateTimeField(default=timezone.now)
      file = models.FileField(upload_to=generate_filename, blank=True)
      template = models.FilePathField(path=generate_templatename, default='DEFAULT')
  
      def __str__(self):
          return self.name
  </code></pre>
  
  <p>(the setting for the file url should be right - i am using the same approach to store files in the /media/documents directory)</p>
  
  <p>How can i combine those two, so that i can choose the model directly? or do i have to set it manually in the view.py later?</p>