Question 187018 in Mongodb Question posted in
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Mongodb – Get all unique values from a collection array object field

I have the collection sessions:

[
    {
      "_id": "",
      "members": [
        { "user_id": "11", "joined_at": "2023-06-15T04:50:06.298Z" },
        { "user_id": "23", "joined_at": "2023-06-15T04:50:06.298Z" },
        { "user_id": "73", "joined_at": "2023-06-15T04:50:06.298Z" }
      ]
    },
    {
      "_id": "",
      "members": [
        { "user_id": "59", "joined_at": "2023-06-15T04:50:06.298Z" },
        { "user_id": "37", "joined_at": "2023-06-15T04:50:06.298Z" },
        { "user_id": "11", "joined_at": "2023-06-15T04:50:06.298Z" }
      ]
    },
    {
      "_id": "",
      "members": [
        { "user_id": "11", "joined_at": "2023-06-15T04:50:06.298Z" },
        { "user_id": "23", "joined_at": "2023-06-15T04:50:06.298Z" },
        { "user_id": "28", "joined_at": "2023-06-15T04:50:06.298Z" }
      ]
    }
]

I want to get all distinct user_id values from the entire collection (or based on a filter). Based on the documents above, the intended results should be:

["11", "23", "73", "59", "37", "28"]

I know that the collection.distinct() function exists, but I’d like to see how it’s done for objects fields in an array.

Tags:

2

Answers


  1. 0

    1. $group – Group all documents and add the members.user_id array into the user_ids array. This results in the user_ids being a nested array.

    2. $set – Set user_ids field.

      2.1. $setUnion – Union the elements from the result 2.1.1. This results in the elements are distinct.

      2.1.1. $reduce – Flatten the nested array user_ids.

    db.collection.aggregate([
  {
    $group: {
      _id: null,
      user_ids: {
        $addToSet: "$members.user_id"
      }
    }
  },
  {
    $set: {
      user_ids: {
        $setUnion: {
          $reduce: {
            input: "$user_ids",
            initialValue: [],
            in: {
              $concatArrays: [
                "$$value",
                "$$this"
              ]
            }
          }
        }
      }
    }
  }
])

    Demo @ Mongo Playground

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  2. 0 
    /*
  Expected Output:
  [
    {
      members: ['11', '23', '73', '59', '37', '28'],
    },
  ]

  step 1: unwind members
  step 2: group by null and add to set members.user_id
  step 3: project members
  
*/

db.collection.aggregate([
  {
    $unwind: '$members',
  },
  {
    $group: {
      _id: null,
      members: {
        $addToSet: '$members.user_id',
      },
    },
  },
  {
    $project: {
      _id: 0,
      members: 1,
    },
  },
]);
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