title: Django admin actions update foreign key
body: <p>I have this simple action I added in my admin.py file.</p>
  
  <pre><code>def groupA(modeladmin, request, queryset):
      queryset.update(group=1)
  groupA.short_description = "Group A"
  
  //
  class StudentAdmin(admin.ModelAdmin):
  ...
  actions = [groupA]
  </code></pre>
  
  <p>So far everything works as expected.</p>
  
  <p>The problem is that group is a foreign key and it would be more manageable if I could do something like</p>
  
  <pre><code>def groupA(modeladmin, request, queryset):
      queryset.update(group="A")
  groupA.short_description = "Group A"
  </code></pre>
  
  <p>I tried with the queryset below but no luck.</p>
  
  <pre><code>     queryset.update(group__name="A")
  </code></pre>
  
  <p>My group model looks like something like that :</p>
  
  <pre><code>class Group(models.Model):
      name = models.CharField(max_length=5, default='', blank=True)
      ....
  
      def __str__(self):
          return self.name
  </code></pre>
  
  <p>So is it possible to do admin actions with the <strong>str</strong> representation of my Group model ?</p>