code: def migrate(pool, from_connection, to_connection):
      
      f = connect_database(from_connection)
      t = connect_database(to_connection)
  
      if isinstance(f, ProjectDB):
          for each in f.get_all():
              each = unicode_obj(each)
              logging.info("projectdb: %s", each['name'])
              t.drop(each['name'])
              t.insert(each['name'], each)
      elif isinstance(f, TaskDB):
          pool = Pool(pool)
          pool.map(
              lambda x, f=from_connection, t=to_connection: taskdb_migrating(x, f, t),
              f.projects)
      elif isinstance(f, ResultDB):
          pool = Pool(pool)
          pool.map(
              lambda x, f=from_connection, t=to_connection: resultdb_migrating(x, f, t),
              f.projects)
docs: Migrate tool for pyspider