MongoDB match array based on document value rather than explicit condition

Question

Let's say I have the following document structure:

{
  "A": {
    "_id": "ID0"
  },
  "B": [
    {
      "_id": "ID0",
      "field": "X"
    },
    {
      "_id": "ID1",
      "field": "Y"
    }
  ]
}

I want to project B matched with the _id in A. The end result would be:

{
  "B": [
    {
      "_id": "ID0",
      "field": "X"
    }
  ]
}

I tried the following but apparently I'm doing something wrong. Is it possible to match based on a document filed rather than an explicit condition?

db.collection.aggregate([
  {$match: {"B._id": "$A._id"}},
  {$project: {"B": 1}}
])

I just made up that example so I might have missed a character somewhere. Feel free to point it out! — CHRD
– CHRD, Commented Apr 15, 2020 at 9:32
B is an array... 'a' and 'b' are fields of same object? two different objects? Not a typo, but a structure issue — matthPen
– matthPen, Commented Apr 15, 2020 at 9:52
"B": [ {"a": { "_id": "ID0", "field": "X" }}, { "b": { "_id": "ID1", "field": "Y" }} ] — matthPen
– matthPen, Commented Apr 15, 2020 at 9:56

matthPen · Accepted Answer · 2020-04-15 10:08:54Z

1

You can use $filter aggregation operator to achieve this :

db.collection.aggregate([
  {
    $addFields: {
      "B": {
        $filter: {
          input: "$B",
          as: "arr",
          cond: {
            $eq: [
              "$A._id",
              "$$arr._id"
            ]
          }
        }
      }
    }
  }
])

Test it here

answered Apr 15, 2020 at 10:08

matthPen

4,3731 gold badge18 silver badges18 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

MongoDB match array based on document value rather than explicit condition

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related