title: using prefetch_related to get only a particular entry of a relation
body: <p>Assume the following models</p>
  
  <pre><code>Class A(): pass
  Class B():
      i = integer()
      aa = foreignkey('A', related_name = 'fka')
  </code></pre>
  
  <p>For simplicity assume the below entries,</p>
  
  <pre><code>A() - a1 -&gt; pk = 1
  B() - b1 -&gt; i = 1, aa = a1
  B() - b2 -&gt; i = 2, aa = a1
  B() - b3 -&gt; i = 3, aa = a1
  B() - b4 -&gt; i = 4, aa = a1
  B() - b5 -&gt; i = 5, aa = a1
  </code></pre>
  
  <p>I know,</p>
  
  <pre><code>foo = A.objects.get(pk = 1).prefetch_related('fka')
  </code></pre>
  
  <p>will give me the entries b1, b2, b3, b4 and b5.</p>
  
  <p>But what I want to know is, is it possible to alter this query in anyway to get only a particular entry of B() associated with A().
  Suppose I want to prefetch only entry of 'B', with 'i' as 2 (get b2, and skip b1, b3, b4 and b5).</p>
  
  <p>If it is not possible to do it using the prefetch then, what is the best way to do it ?</p>
  
  <p>Note: <code>get(pk = 1)</code> is used to keep the simplicity for explanation, but at that place usually there will be <code>filter(**conditions)</code>.</p>