title: Django objects.filter multiple filters
body: <p>The user can select fields to filter by, I need to filter by only those fields but there can be 3 fields.</p>
  
  <p>The user selects all three options:a, b and c</p>
  
  <pre><code>Foo.objects.filter(a=1,b=2,c=3), good
  </code></pre>
  
  <p>What if the user selects only 1 option or 2 options ?</p>
  
  <pre><code>Foo,objects.filter(a=1, b=2, c=not selected)
  </code></pre>
  
  <p>How can I do this to only filter by the selected choices. THis comes from a post to the view, and looks like this if not selected:</p>
  
  <pre><code>a=1,b=NaN,c=3
  </code></pre>
  
  <p>So b was not selected and I would not like to inlclude it in my filter,</p>
  
  <pre><code>Foo.objects.filter(a=1,c=3)
  </code></pre>
  
  <p>Or can I perhaps so a filter that is basically a "all" selector</p>
  
  <p>So as per above:</p>
  
  <pre><code>Foo.objects.filter(a=1,b=%,c=3)
  </code></pre>