Question

There are two sets:
A set of “user information”;
“Articles” are stored in set B;

Each article in collection b has a comment field as follows:

comment:[{
 uid:"59ffd1966e39780ae3fd99c3",
 Message:' Comments'
 },{
 uid:"59ffd269fbcba4346517c098",
 Message:' Comments'
 },{
 uid:"59ffd278fbcba4346517c099",
 Message:' Comments'
 }]

How to use uid to associate the user information in set A when querying article comments? The expected results are as follows:

comment:[{
 Username: "User Name",
 
 ......
 
 Message:' Comments'
 },{
 Username: "User Name",
 
 ......
 
 Message:' Comments'
 },{
 Username: "User Name",
 
 ......
 
 Message:' Comments'
 }]

It is not desirable to find out and reinsert the data of set A when inserting data into set B, thus there is data redundancy. . . . ;
Only the user ID and no other user information are put in the B set.

Looking through MONGODB MANUAL, I could not find it. . . .

db.b.aggregate([
 {$unwind: "$comment"},
 {
 $lookup:
 {
 from: "A",
 localField: "comment.uid",
 foreignField: "uid",
 as: "guest"
 }
 },
 {$group : {_id:"$title",comments:{$push:"$comment"}}
 ]);

In fact, the only results returned are title and comments, and all other related fields are missing. . . .

Solve!

Answer 1

db.b.aggregate([
 {$unwind: "$comment"},
 {
 $lookup:
 {
 from: "A",
 localField: "comment.uid",
 foreignField: "uid",
 as: "guest"
 }
 },
 {$ group: {_ id: "$ title", fulldoc: {$ push: "$ $ root"}}/<-put the entire document in
 ]);

Although this can achieve your goal, it is suggested that you still consider redundancy. Data consistency here is not so important.
In addition$lookupAlthough it can solve the problem, its performance is not very good, and it does not support Sharding, so it should be used with caution.