title: Check if Django model field choices exists
body: <p>I'm attempting to check if a value exists in the choices tuple set for a model field.  </p>
  
  <p>For example lets say I have a Model like this:</p>
  
  <pre><code>class Vote(models.Model):
   VOTE_TYPE = (
    (1, "Up"),
    (-1, "Down"),
   )
  
   value = models.SmallIntegerField(max_length=1, choices=VOTE_TYPES)
  </code></pre>
  
  <p>Now lets say in a view I have a variable <code>new_value = 'Up'</code> that I would like to use as the value field in a new Vote.  How can I first check to see if the value of that variable exists in the VOTE_TYPE tuple?  Thank you.</p>