title: How to execute "left outer join" in SqlAlchemy
body: <p>I need to execute this query::</p>
  
  <pre><code>select field11, field12
  from Table_1 t1
  left outer join Table_2 t2 ON t2.tbl1_id = t1.tbl1_id
  where t2.tbl2_id is null
  </code></pre>
  
  <p>I had these classes in python:</p>
  
  <pre><code>class Table1(Base):
     ....
  
  class Table2(Base):
     table_id =  Column(
          Integer,
          ForeignKey('Table1.id', ondelete='CASCADE'),
      )
      ....
  </code></pre>
  
  <p>How do I get to the above from the below?</p>