$lookupOperator can do left outer join. This is the only connection currently supported by MongoDB. If you need a large number of such tables in your program, it usually means that the data model is designed according to the relational model. In this case, the data model should be modified earlier, or the future trouble will be endless.
If you look up the information, you may find another one.DBRef. It is not recommended to use, the efficiency is relatively poor, that is, the driver helped you to do an additional query operation.