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I am new to mongodb so I hope this does not come-off as a very elementary question. I've done some research and tried to apply what I've found but something just seems to escape me.

I have two collections of the following format:

-----------------------------------------------------------------------
-----------------------------------------------------------------------
    "shopId": "1002",
    "shopPosId": "10002",
    "description": "some description"
-----------------------------------------------------------------------
Compte
-----------------------------------------------------------------------
    "shopId": "9000",
    "shopPosId": "0000",
    "clientUid": "474192"
I want to join those and before doing so, I want to filter out the Shops which do not have the shopPosId field. 
Here's my code: 
Compte.aggregate([
        $match:
                $and:[{"clientUid":clientUid}]
        $lookup:
            from: "Shop",
            localField: "shopId",
            foreignField: "shopId",
            let: {"Shop.shopPosId": "$shopPosId"},
            pipeline: [{$match: {"shopPosId": {"$exists": false}}}],
            as: "shopDescr"
the returned result is an undefined, which means the query doesn't make much sense (because in fact I should at least get a void array). 
Is this because the two collections have the shopPosId field? (if so, isn't this line let: {"Shop.shopPosId": "$shopPosId"} supposed to take care of it ?)
                You are mixing $lookup syntax. It is either from-localField-foreignField-as or from-let-pipeline-as, but not both together.
– Alex Blex
                Jul 3, 2018 at 15:45
As Alex commented you are mixing both the  $lookup syntax here... So the correct will be
Compte.aggregate([
  { "$match": { "$and": [{ "clientUid": clientUid }] }},
  { "$lookup": {
    "from": "Shop",
    "let": { "shopId": "$shopId" },
    "pipeline": [
      { "$match": {
        "$expr": { "$eq": [ "$shopId", "$$shopId" ] },
        "shopPosId": { "$exists": false }
    "as": "shopDescr"
                Ok, this is informative, thanks! Still, I get an undefined result, I'd assume it's to do with my data except I really don't think so. Is it necessary to have mongoDB 3.6 to be able and apply a pipeline in a lookup ?
– KitKatKot
                Jul 4, 2018 at 8:46
                Yeah, seems like it is, confirmed in the officiel documentation: docs.mongodb.com/manual/reference/operator/aggregation/lookup/…
– KitKatKot
                Jul 4, 2018 at 9:03
        
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