How do I query the 2 collections in mongoose?

Question

I have 2 collections namely eventPart and Participants.

eventPart:{
   {
    "_id" : ObjectId("5b62676ba5419449046a26c4"),
    "rollNo" : "15K21A0528",
    "eventName" : "Ebullienza",
    "team" : 68,
     "__v" : 0
},{
    "_id" : ObjectId("5b62676ba5419449046a26c5"),
    "rollNo" : "15G11A0518",
    "eventName" : "Ebullienza",
    "team" : 68,
    "__v" : 0}
}

Participants : {
   {
     "_id" : ObjectId("5b62626d9306a610e36aca95"),
     "rollNo" : "15S11A0528",
     "name" : "Pavan Boro",
     "email" : "pavan@gmail.com",
     "mobile" : NumberLong(6700332950),
     "branch" : "CSE",
     "section" : "A",
     "year" : 4
}, {
     "_id" : ObjectId("5b62633d9306a610e36acae1"),
     "rollNo" : "15S11A0518",
     "name" : "Manoj",
     "email" : "manoj@gmail.com",
     "mobile" : NumberLong(9700332910),
     "branch" : "CSE",
     "section" : "A",
     "year" : 4
}}

I want to query the database with eventName in eventPart along with data in Participant collection.

Ex: With eventName : 'Ebullienza' I want to fetch all the details that is related to eventName: 'Ebullienza' in Participants collection.

I have tried collecting the rollNo from eventPart and using it for in Participants collection. But I get stuck in collecting the rollNo in array

 var rollNumber = new Array();
 EventPart.find({'eventName':'Ebullineza'}).then(function(a){
    a.forEach(function(roll){
        rollNumber.push(roll.rollNo);
  });
});
console.log(rollNumber); // prints [] - empty array

rollNumber is printed as [].

I think there could be a more efficient way of query 2 collections in mongoose.

Thanks.

which is matching key in both the collections and what is your mongodb version? — Ashh, Aug 02 '18 at 13:46
Possible dupe of https://stackoverflow.com/questions/35813854/how-to-join-multiple-collections-with-lookup-mongodb — s7vr, Aug 02 '18 at 13:58
Try `EventPart.aggregate([ { $lookup: { from: "participants", localField: "rollNo", foreignField: "rollNo", as: "participants" } })` — s7vr, Aug 02 '18 at 14:01

score 1 · Answer 1 · answered Aug 02 '18 at 14:01

use $lookup

db.eventPart.aggregate([
    { "$match": { "eventName": "Ebullienza" } },
    {
        "$lookup": {
            "from": "Participants",
            "localField": "rollNo",
            "foreignField": "rollNo",
            "as": "Participants"
        }
    }
])

score 0 · Answer 2 · answered Aug 02 '18 at 17:46

print your array in the callback... otherwise he is printed before the end of execution of your find

 var rollNumber = new Array();
 EventPart.find({'eventName':'Ebullineza'}).then(function(a){
    a.forEach(function(roll){
        rollNumber.push(roll.rollNo);
  });
  console.log(rollNumber); // prints [] - empty array
});

How do I query the 2 collections in mongoose?

2 Answers2