title: build When Case query in for loop django
body: <p>i try build <code>queryset</code> for update data with <code>When Case</code>. It only can build after one <code>for</code> loop with dynamic param. </p>
  
  <p>Example i have model <code>Employee</code>:</p>
  
  <pre><code>class Employee(models.Model):
      account_type = models.TextField(blank=True,  null=True)
  </code></pre>
  
  <p>i want update it like this, if id = 1 then value = 1, if value = 2 then value = 2</p>
  
  <pre><code>Employee.objects.update(
       account_type=Case(
           When(id=1,
                then=Value("1")),
          When(id=2,
                then=Value("2")),
       ),
   )
  </code></pre>
  
  <p>but i want <code>id</code> is dynamic, so i cant build static queryset. I want build <code>Case</code> in one <code>for loop</code>.
  My ideal like this:</p>
  
  <pre><code>final_case = None
  for one_id in list_id:
     final_case += Case(When(id=one_id, then=Value(one_id)))
  </code></pre>
  
  <p>I want build <code>final_case</code> and update only like </p>
  
  <pre><code>Employee.objects.update(account_type=final_case)
  </code></pre>
  
  <p>Anybody know how to do it ? Thanks</p>