title: filter by row_number in sqlalchemy
body: <p>how can I filter the row_number ==1 in the following query:</p>
  
  <pre><code>query = session.query(Foo, func.row_number().over(
          partition_by=Foo.foo_field, order_by=desc(Foo.foo_date_time)).label("row_number"))
  
      query = query.filter(Foo.time_key &lt;= time_key)
      # query = query.filter(row_number == 1)
      result = query.all()
  </code></pre>