code: def get_node_selectable(node, context):
      
      query_path = node.query_path
      if query_path not in context.query_path_to_selectable:
          raise AssertionError(
              u'Unable to find selectable for query path {} with context {}.'.format(
                  query_path, context))
      selectable = context.query_path_to_selectable[query_path]
      return selectable
docs: Return the Selectable Union[Table, CTE] associated with the node.