title: Propel equivalent of "exists"
body: <p>I am new to Propel and have been reading the documentation.  But, I have not found a clear equivalent to the EXISTS and NOT EXISTS constructs from SQL.  Linq in .NET, for instance, has Any().  Is there an equivalent to the following in "idiomatic" Propel?</p>
  
  <pre><code>SELECT a.column1, a column2, a.etc
  FROM   TableA a
  WHERE  NOT EXISTS (SELECT 1
                     FROM   TableB b
                     WHERE  b.someIdColumn = a.someIdColumn
                     AND    b.aNullableDateColumn IS NULL)
  </code></pre>