title: django custom field in admin inline
body: <p>Here's admin.py:</p>
  
  <pre><code>class AnsweredQuestionInline(ReadonlyTabularInline):
      model = AnsweredQuestion
      fields = ('question', 'selected_choice', 'one_answer')
  
      def one_answer(self, obj):
          return obj.get_answer()
  
  
  class CompletedTestAdmin(admin.ModelAdmin):
      inlines = [AnsweredQuestionInline]
  </code></pre>
  
  <p>When I go to completed test admin interface I get this error:</p>
  
  <pre><code>Exception Type: FieldError at /admin/tests/completedtest/5/
  Exception Value: Unknown field(s) (one_answer) specified for AnsweredQuestion
  </code></pre>
  
  <p>Is it possible to add custom fields for django's inline admin ? If so what am I doing wrong ?</p>