title: How to filter Django's CommaSeparatedIntegerField
body: <p>Assume a model</p>
  
  <pre><code>class Foo(models.Model):
      bar = models.CommaSeparatedIntegerField('Filter Me!')
  </code></pre>
  
  <p>The content of the <code>bar</code> could look like, e.g., <code>12,35,67,142</code>.</p>
  
  <p>I want to query all <code>Foo</code>s, that have a <code>42</code> in <code>bar</code>:</p>
  
  <pre><code>all_42_foos = Foo.objects.filter(bar__contains="42")
  </code></pre>
  
  <p>which doesn't give the correct result, since <code>CommaSeparatedIntegerField</code> inherits from <code>CharField</code> and the filter evaluation uses the string content of the field (matching the above example with <code>142</code>, too).</p>
  
  <p>How can I hav a filter, that does a <code>.split(",")</code> on the <code>bar</code> field before it checks for the <code>42</code>? I really don't want <code>bar</code> to become a <code>ManyToMany</code>, since it'd be a terrible overhead.</p>