Fix bug with related

Before I made this fix, I experienced this error when starting my app:

      File "/usr/local/lib/python2.7/site-packages/django/db/models/query.py", line 590, in filter
        return self._filter_or_exclude(False, *args, **kwargs)
      File "/usr/local/lib/python2.7/site-packages/django/db/models/query.py", line 608, in _filter_or_exclude
        clone.query.add_q(Q(*args, **kwargs))
      File "/usr/local/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1198, in add_q
        clause = self._add_q(where_part, used_aliases)
      File "/usr/local/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1234, in _add_q
        current_negated=current_negated)
      File "/usr/local/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1100, in build_filter
        allow_explicit_fk=True)
      File "/usr/local/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1357, in setup_joins
        names, opts, allow_many, allow_explicit_fk)
      File "/usr/local/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1265, in names_to_path
        field, model, direct, m2m = opts.get_field_by_name(name)
      File "/usr/local/lib/python2.7/site-packages/django/db/models/options.py", line 373, in get_field_by_name
        cache = self.init_name_map()
      File "/usr/local/lib/python2.7/site-packages/django/db/models/options.py", line 404, in init_name_map
        for f, model in self.get_all_related_objects_with_model():
      File "/usr/local/lib/python2.7/site-packages/django/db/models/options.py", line 466, in get_all_related_objects_with_model
        self._fill_related_objects_cache()
      File "/usr/local/lib/python2.7/site-packages/django/db/models/options.py", line 494, in _fill_related_objects_cache
        cache[f.related] = None
    AttributeError: 'CustomForeignKey' object has no attribute 'related'