title: sqlalchemy: Order query after UNION
body: <p>I have two different tables but both tables have a date object. I would like to query both tables at the same time to get a mixed list of objects ordered by date.
  I need to do this in one step, so that I can use the paginate command. I cannot just query two lists and combine them afterwards.</p>
  
  <p>UPDATE: I tried</p>
  
  <pre><code>dummy = query1.union(query2).order_by('date').all()
  </code></pre>
  
  <p>but that results in </p>
  
  <pre><code>ArgumentError: All selectables passed to CompoundSelect must have identical numbers of columns; select #1 has 8 columns, select #2 has 24
  </code></pre>
  
  <p>thanks
  carl</p>