How to join query in mongodb?

Question

I have user document collection like this:

User {
   id:\"001\"
   name:\"John\",
   age:30,
   friends:[\"userId1\",\"userId2\",\"userId3\"....]
}
         


        

Answer 1

one kind of join a query in mongoDB, is ask at one collection for id that match , put ids in a list (idlist) , and do find using on other (or same) collection with $in : idlist

u = db.friends.find({"friends": ? }).toArray()
idlist= []
u.forEach(function(myDoc) { idlist.push(myDoc.id ); } )
db.friends.find({"id": {$in : idlist} } )