title: Custom arithmetic filter clause in ORM
body: <p>I am trying to retrieve from a particular table all rows for which a certain subtraction results in a value smaller or equal to a certain fixed value. In SQL it would look like this:</p>
  
  <p><code>SELECT * FROM my_table WHERE column_a - column_b &lt;= 1;</code></p>
  
  <p>In Django, I thought I could apply a <code>filter</code> combining <code>F</code> expressions, but as far as I have seen that is only possible if the value to compare against is a column or annotation. Since usage of the the <code>extra</code> keyword is discouraged and <code>RawSQL</code> is also not an attractive option, I would like to know if there is a built-in way to apply this kind of filtering. Of course I could also do with an annotation, but out of curiosity would be good to know if there is a more 'direct' way.</p>
  
  <p>Thanks in advance!</p>