def update_display_field(self):
        display_field = self.category.display_field
        if display_field is not None:
            value = None
            if self.properties:
                value = self.properties.get(display_field.key)

            self.display_field = '%s:%s' % (display_field.key, value)
Updates the display_field attribute. It uses the display field of the
dataset/ETHPy150Open ExCiteS/geokey/geokey/contributions/models.py/Observation.update_display_field