title: django: how to change values for nullbooleanfield in a modelform?
body: <p>In the docs, the <code>nullbooleanfield</code> is represented <code>as a &lt;select&gt; box with "Unknown", "Yes" and "No" choices.</code> How can I change the values of <code>select</code> to some other more meaningful texts and map it back to the yes,no and unknown values in my <code>modelform</code>?</p>
  
  <p>For example I have <code>yes_no_required = models.NullBooleanField()</code> and I would like to have <code>'yes I acknowledge this'</code>, <code>'no, I do not like this'</code> and <code>'I do not know now'</code> mapping to yes, no and required accordingly.</p>