title: Can not query the json column with sqlachemy
body: <p>I am trying to query with in json column in postgresql with <code>flask-sqlalchemy</code>.
  here is my code</p>
  
  <pre><code>house_ = House()
  results = house_.query.filter(
              House.information['branch_name'].astext == 'release0'
              ).all()
  </code></pre>
  
  <p>I am not sure what is wrong.
  I tried to use <code>.cast(Unicode)</code> instead of <code>astext</code> as well.</p>
  
  <p>Getting error as below:</p>
  
  <pre><code>NotImplementedError: Operator 'getitem' is not supported on this expression
  </code></pre>