title: Django annotate() error AttributeError: 'CharField' object has no attribute 'resolve_expression'
body: <p>Hello I want to concatenate more fields into django, but even this simple code:</p>
  
  <pre><code>    Project.objects.annotate(
          companyname=Concat('company__name',Value('ahoj')),output_field=CharField()    
      )
  </code></pre>
  
  <p>Gives me an error:</p>
  
  <pre><code>AttributeError: 'CharField' object has no attribute 'resolve_expression'
  </code></pre>
  
  <p>Traceback:</p>
  
  <pre><code>  File "/root/MUP/djangoenv/lib/python3.4/site-packages/django/db/models/manager.py", line 122, in manager_method
      return getattr(self.get_queryset(), name)(*args, **kwargs)
    File "/root/MUP/djangoenv/lib/python3.4/site-packages/django/db/models/query.py", line 908, in annotate
      clone.query.add_annotation(annotation, alias, is_summary=False)
    File "/root/MUP/djangoenv/lib/python3.4/site-packages/django/db/models/sql/query.py", line 986, in add_annotation
      annotation = annotation.resolve_expression(self, allow_joins=True, reuse=None,
  AttributeError: 'CharField' object has no attribute 'resolve_expression'
  </code></pre>