docs: def _get_populate_from_value(self, instance):
          """Get the value from ``populate_from`` attribute."""
          if hasattr(self.populate_from, '__call__'):
              # ResolweSlugField(populate_from=lambda instance: ...)
              return self.populate_from(instance)
          else:
              # ResolweSlugField(populate_from='foo')
              attr = getattr(instance, self.populate_from)
              return attr() if callable(attr) else attr