title: Sqlalchemy select condition
body: <p>I have a mapper on a table and I want to define a column_property which should select True or False whether the entity has some propertie or not:</p>
  
  <pre><code>mapper( Person, persons_table, properties = {
      'administrator': column_property( 
          select( 
              [True if roles_table.c.is_admin or roles_table.c.id == 1 else False],
              roles_table.c.id == persons_table.c.role_id
          ).label( 'administrator' )
          )
  } )
  </code></pre>
  
  <p>Is this something I can do? I'm more interested in this part: <code>[True if roles_table.c.is_admin or roles_table.c.id == 1 else False],</code> which lets me set a value to the column based on the condition.</p>