code: def _get_flat_db_sources(self, model):
          
          sources = []
          for source in self.sources:
              for sub_source in self.expand_source(source):
                  target_field = self.resolve_source(model, sub_source)
                  if target_field:
                      sources.append(sub_source)
          return sources
docs: Return a flattened representation of the individual ``sources`` lists.