title: django error __str__ returned non-string (type __proxy__)
body: <p>I have some models that use <strong>GenericForeignKey</strong>, and when I try to access them with Django Admin util, I can see the list of records, but when I click on one of them, I get this error: <code>__str__ returned non-string (type __proxy__)</code>.</p>
  
  <p>This is the code of one of the models:</p>
  
  <pre><code>class ReservationComponent(models.Model):
      reservation = models.ForeignKey(Reservation, on_delete=models.PROTECT, related_name='components', verbose_name=_(''))
      day = models.DateField(verbose_name=_('Day'))
      content_type = models.ForeignKey(ContentType, on_delete=models.PROTECT)
      object_id = models.PositiveIntegerField()
      content_object = GenericForeignKey('content_type', 'object_id')
  
      class Meta:
          verbose_name = _("Reservation Component")
          verbose_name_plural = _("Reservations Components")
  
      def __str__(self):
          return "[{}][{}]{} - [{}]{}".format(self.id, self.reservation, self.day, self.content_type, self.object_id)
  </code></pre>
  
  <p>This is the whole traceback:</p>
  
  <pre><code>Internal Server Error: /es/admin/ReservationsManagerApp/reservationcomponent/14/change/
  Traceback (most recent call last):
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/core/handlers/exception.py", line 35, in inner
      response = get_response(request)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/core/handlers/base.py", line 158, in _get_response
      response = self.process_exception_by_middleware(e, request)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/core/handlers/base.py", line 156, in _get_response
      response = response.render()
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/response.py", line 106, in render
      self.content = self.rendered_content
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/response.py", line 83, in rendered_content
      content = template.render(context, self._request)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/backends/django.py", line 61, in render
      return self.template.render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 175, in render
      return self._render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 167, in _render
      return self.nodelist.render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 943, in render
      bit = node.render_annotated(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 910, in render_annotated
      return self.render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/loader_tags.py", line 155, in render
      return compiled_parent._render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 167, in _render
      return self.nodelist.render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 943, in render
      bit = node.render_annotated(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 910, in render_annotated
      return self.render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/loader_tags.py", line 155, in render
      return compiled_parent._render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 167, in _render
      return self.nodelist.render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 943, in render
      bit = node.render_annotated(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 910, in render_annotated
      return self.render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/loader_tags.py", line 67, in render
      result = block.nodelist.render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 943, in render
      bit = node.render_annotated(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 910, in render_annotated
      return self.render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/loader_tags.py", line 67, in render
      result = block.nodelist.render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 943, in render
      bit = node.render_annotated(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 910, in render_annotated
      return self.render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/defaulttags.py", line 211, in render
      nodelist.append(node.render_annotated(context))
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 910, in render_annotated
      return self.render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/loader_tags.py", line 194, in render
      return template.render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 177, in render
      return self._render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 167, in _render
      return self.nodelist.render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 943, in render
      bit = node.render_annotated(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 910, in render_annotated
      return self.render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/defaulttags.py", line 211, in render
      nodelist.append(node.render_annotated(context))
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 910, in render_annotated
      return self.render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/defaulttags.py", line 211, in render
      nodelist.append(node.render_annotated(context))
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 910, in render_annotated
      return self.render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/defaulttags.py", line 314, in render
      return nodelist.render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 943, in render
      bit = node.render_annotated(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 910, in render_annotated
      return self.render(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 993, in render
      output = self.filter_expression.resolve(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 676, in resolve
      obj = self.var.resolve(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 802, in resolve
      value = self._resolve_lookup(context)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 864, in _resolve_lookup
      current = current()
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/contrib/admin/helpers.py", line 137, in label_tag
      contents = conditional_escape(self.field.label)
    File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/utils/html.py", line 78, in conditional_escape
      text = str(text)
  TypeError: __str__ returned non-string (type __proxy__)
  [22/Aug/2018 02:52:14] "GET /es/admin/ReservationsManagerApp/reservationcomponent/14/change/ HTTP/1.1" 500 489781
  </code></pre>