How to perform multiple $lookup for array mongodb aggregate

Question

How can i perform $lookup in aggregate (mongodb) for an array

{
     messages: [{
            "_id" : ObjectId("5bfc43f2bbc4176ecc1c5f83"),
            "text" : "text1",
            "sender" : {
                    "id" : "36046fc2e70dd508a0bf1f36fd2daa20"
            }
         }, {
            "_id" : ObjectId("5bfc43f2bbc4176ecc1c5f83"),
            "text" : "text2",
            "sender" : {
                    "id" : "36046fc2e70dd508a0bf1f36fd2daa22"
            }
    }],
    "filed1": { ... },
    "filed2": { ... }
}

how can i do a $lookup for sender id from accounts collection?

tried:

...
      {
        $lookup: {
          from: "accounts",
          localField: "messages.sender.id",
          foreignField: "id",
          as: "messages.sender.user"
        }
      }
...

Not sure what you are asking. But if the below answer works for you then please accept it and If you have any further doubts then please ask a new question in separate thread. — Ashh
– Ashh, Commented Nov 30, 2018 at 7:33
I can't create new thread but its ok Why? I will surely help you. — Ashh
– Ashh, Commented Nov 30, 2018 at 7:37
Pushing all the document in a single array in a $group stage is not a good job. It will breach the BSON limit of 16 MB. I have upvoted your one question try now to ask. — Ashh
– Ashh, Commented Nov 30, 2018 at 7:45
I cannot understand. Please edit your question and put the whole explanation to it. — Ashh
– Ashh, Commented Nov 30, 2018 at 7:48

Ashh · Accepted Answer · 2018-11-30 07:53:01Z

2

You can use below aggregation

db.collection.aggregate([
  { "$unwind": "$messages" },
  { "$lookup": {
    "from": "accounts",
    "localField": "messages.sender.id",
    "foreignField": "id",
    "as": "messages.sender.user"
  }},
  { "$unwind": "$messages.sender.user" }
  { "$group": {
    "_id": "$_id",
    "messages": { "$push": "$messages" },
    "filed1": { "$first": "$filed1" },
    "filed2": { "$first": "$filed2" }
  }}
])

edited Nov 30, 2018 at 7:53

answered Nov 27, 2018 at 2:35

Ashh

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