Aggregate nested objects mongodb

Question

Could you please help me to write some sort of aggregation query using mongodb.

I have next data structure.

[
    {
        id: 1,
        shouldPay: true,
        users: [
            {
                id: 100,
                items: [{...}],
                tags: ['a', 'b', 'c', 'd']
            },
            {
                id: 100,
                items: [{...}],
                tags: ['b', 'c', 'd']
            },
            {
                id: 100,
                items: [{...}],
                tags: ['c', 'd']
            }
        ],

    }
]

In result I want to get something like that:

[
    {
        id: 1,
        shouldPay: true,
        user: {
            id: 100,
            items: [{...}],
            tags: ['a', 'b', 'c', 'd']
        }

    }
]

The main idea is to select a specific user that has "a" letter or list of letters ['a', 'b'] in tags.

@astro noup, that's another case – Bob Dylan Nov 18 '18 at 13:03 — Bob Dylan, Nov 18 '18 at 13:03

Ashh · Accepted Answer · 2018-11-18T15:33:37.983

2

You can use below aggregation

Use $match at the starting of the pipeline to filter out the documents which don't contain "a" and "b" in tags array. And then use $filter with $setIsSubset to filter out the nested array.

$arrayELemAt to return the specified element from the array.

db.collection.aggregate([
  { "$match": { "users.tags": { "$in": ["a", "b"] }}},
  { "$project": {
    "users": {
      "$arrayElemAt": [
        { "$filter": {
          "input": "$users",
          "cond": { "$setIsSubset": [["a", "b"], "$$this.tags"] }
        }},
        0
      ]
    }
  }}
])

edited Nov 18 '18 at 15:33

answered Nov 18 '18 at 14:48

Ashh

44,693
14
105
132

good, but your code will fail in case if we will start matching with an array of elements. let's imagine that you will have something like this { "$in": [['a', 'b', 'c'], "$$this.tags"] } – Bob Dylan Nov 18 '18 at 15:05
1

You haven't mentioned that in question. Please clarify your question. Do want to match with single element or with an array – Ashh Nov 18 '18 at 15:09
sorry about that – Bob Dylan Nov 18 '18 at 15:23
Updated my answer – Ashh Nov 18 '18 at 15:26
pretty good idea to use $setIsSubset, but I tried to use it previously, and it doesn't work with the nested objects. – Bob Dylan Nov 18 '18 at 15:41
Don't know how did you use that. What's now ? Take a [look](https://mongoplayground.net/p/Gu-SLfdnZr0) – Ashh Nov 18 '18 at 15:44
1

thanks, yes it works – Bob Dylan Nov 18 '18 at 15:51
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/183852/discussion-between-bob-dylan-and-anthony-winzlet). – Bob Dylan Nov 18 '18 at 15:54

score 0 · Answer 2 · answered Nov 18 '18 at 13:42

0

You need to use $unwind along with $filter:

db.collection.aggregate([
      {
        $unwind: "$users"
      },
      {
        $match: {
          "users.tags": "a"
        }

      }
    ])

Result:

[
  {
    "_id": ObjectId("5a934e000102030405000000"),
    "id": 1,
    "shouldPay": true,
    "users": {
      "id": 100,
      "tags": [
        "a",
        "b",
        "c",
        "d"
      ]
    }
  }
]

answered Nov 18 '18 at 13:42

Atish

4,277
2
24
32

not the best idea, as can be the situation that you had more than 20k records and unwind produce a big amount additional records per document – Bob Dylan Nov 18 '18 at 14:10

Aggregate nested objects mongodb

2 Answers2