title: Web2Py - How to make field enabled?
body: <p>I have this form that gets built from the <em>SQLFORM</em> method, </p>
  
  <pre><code>flow_step_forms[step.name] = SQLFORM(db.approval_flows_progress_steps,
                                          record=thisstep.id if thisstep is not None else None,
                                          readonly=not __userCanPerformStep(step) or flow_completed)
  </code></pre>
  
  <p>the db for <em>approval_flows_progress_steps</em> is as follows</p>
  
  <pre><code>db.define_table('approval_flows_progress_steps', 
      Field('flow_progress', 'reference approval_flows_progress'),
      Field('flow_step', 'reference approval_flows_steps'),
      Field('completed', 'boolean'),
      Field('completed_on', 'datetime'),
      Field('completed_by', 'reference auth_user'),
      Field('notes', 'text', writable = True), 
      auth.signature
      )
  </code></pre>
  
  <p>the field <em>notes</em> gets rendered as a textbox. </p>
  
  <p>After I assign the SQLFORM there is this code:</p>
  
  <pre><code>if __userCanPerformStep(step) and not flow_completed:
     flow_step_forms[step.name].element('textarea')['_rows'] = 5
  </code></pre>
  
  <p>I figured I can reference properties of the db like they do <a href="http://web2py.com/books/default/chapter/29/07/forms-and-validators#Storing-the-original-filename" rel="nofollow noreferrer">here</a> but I get the following error: 
  <strong> 'NoneType' object has no attribute 'writable'</strong></p>
  
  <p>Q: How can I make the text box conditionally enabled for certain users?? </p>