Mongodb left join query the reason why as is empty

problem description

I have two tables, the user table and the dynamic table. I want to query all the dynamic users while traversing to get the corresponding users
all I use the left join, but the as returned by the query is empty. Why

the environmental background of the problems and what methods you have tried

this is my table
clipboard.png
clipboard.png

clipboard.png


clipboard.png

mongodb compass users s objectId user sex notecontent id

clipboard.png
clipboard.png

Apr.11,2022
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1eb08e0-480b9.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1eb08e0-480b9.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
Need Help?