docs: def register_signals(self):
          """Register signals."""
          from .models import Collection
          from .receivers import CollectionUpdater
  
          if self.app.config['COLLECTIONS_USE_PERCOLATOR']:
              from .percolator import collection_inserted_percolator, \
                  collection_removed_percolator, \
                  collection_updated_percolator
              # Register collection signals to update percolators
              listen(Collection, 'after_insert',
                     collection_inserted_percolator)
              listen(Collection, 'after_update',
                     collection_updated_percolator)
              listen(Collection, 'after_delete',
                     collection_removed_percolator)
          # Register Record signals to update record['_collections']
          self.update_function = CollectionUpdater(app=self.app)
          signals.before_record_insert.connect(self.update_function,
                                               weak=False)
          signals.before_record_update.connect(self.update_function,
                                               weak=False)