title: How do I generate dynamic fields in WTForms
body: <p>I am trying to generate a form in WTForms that has dynamic fields according to this documentation <a href="http://wtforms.simplecodes.com/docs/1.0.2/specific_problems.html#dynamic-form-composition" rel="noreferrer">http://wtforms.simplecodes.com/docs/1.0.2/specific_problems.html#dynamic-form-composition</a></p>
  
  <p>I have this subform class which allows users to pick items to purchase from a list:</p>
  
  <pre><code>class Item(Form):
      itmid = SelectField('Item ID')
      qty = IntegerField('Quantity')
  
  class F(Form):
          pass
  </code></pre>
  
  <p>There will be more than one category of shopping items, so I would like to generate a dynamic select field based on what categories the user will choose:</p>
  
  <pre><code>fld = FieldList(FormField(Item))
  fld.append_entry()
  </code></pre>
  
  <p>but I get the following error:</p>
  
  <pre><code>AttributeError: 'UnboundField' object has no attribute 'append_entry'
  </code></pre>
  
  <p>Am I doing something wrong, or is there no way to accomplish this in WTForms? </p>