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

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".