mongodb string 函数支持$trunc 等操作吗？

db.createView( "view_test", "play_end", [ { $match: { "pv":"1.12.0" } }, { $project: { "time":Date("$t"), "channel":"$mi", "app_version":"$pv", "event_name":"$i", "video_id":"$x.video_id", "clarity":"$x.def", "play_task_id":"$x.play_task_id", "play_time-h":String({"$trunc":{"$divide":["$x.play_time",3600]}})
"video_of_time":"$x.video_of_time", "play_engine_version":"$x.playver", "network_library_version":"$x.netver", "net_error_result":"$x.net_error_result", "net_error_id":"$x.net_error_id" } } ] )

 play_time-h 得到的是 object-object,这个难道不能这样写吗？