MongoDB - Equivalent of LEFT JOIN where one collection isn't exists

Question

Is there an equivalent to LEFT JOIN query where right collection isn\'t exists in MongoDB?

SQL:

SELECT * FROM TableA as A LEFT JOIN TableB as B ON A         


        

Answer 1

Neil Lunn's solution is working, but I have another approach, because $lookup pipe does not support Shard collection in the "from" statement.

So I used to use simple java script as follows. It's simple and easy to modify. But for performance you should have proper indexes!

var mycursor = db.collA.find( {}, {_id: 0, myId:1} ) 

mycursor.forEach( function (x){ 
    var out = db.collB.count( { yourId : x.myId } )
    if ( out > 0) {
        print('The id exists! ' + x.myId); //debugging only

        //put your other query in  here....

        }
} )

Answer 2

Well your edit basically has the answer. Simply $match where the array is empty:

db.getCollection('collA').aggregate([
    { "$lookup": {
      "from": "collB",
      "localField": "_id",
      "foreignField": "_id",
      "as": "collB"
    }},
   { "$match": { "collB.0": { "$exists": false } } }
])

The $exists test on the array index of 0 is the most efficient way to ask in a query "is this an array with items in it".