docs: def add_source(self, source_name, description=None):
          """Adds a row to table "source" if "name" does not
          exist. Returns (source_id, True) if a new row is created,
          (source_id, False) otherwise.
  
          """
  
          # TODO: shoud be able to do this inside a transaction
  
          if not source_name:
              raise ValueError('"source_name" may not be None or an empty string')
  
          sel = select([self.source.c.id], self.source.c.name == source_name).execute()
          result = sel.fetchone()
          if result:
              return result[0], False
          else:
              ins = self.source.insert().execute(
                  name=source_name, description=description)
              return ins.inserted_primary_key[0], True