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....

        }
} )