Hello I am retrieving the following data

[
    {
        "_id": null,
        "count": 2,
        "students": [
            [
                "a",
                "b"
            ],
            [
                "a",
                "c",
                "d"
            ]
        ]
    }
]

Using this querie:

aggregate([
    {
      $match: {
        class: {
          $in: classes,
        },
      },
    },
    {
      $project: { students: 1 },
    },
    {
      $group: {
        _id: null,
        count: { $sum: 1 },
        data: { $push: "$students" },
      },
    },
    {
      $project: {
        count: 1,
        students: {
          $setIntersection: "$data",
        },
      },
    },
  ])

But the idea is that students become at an intersection of that arrays in order to get only

 [
    {
        "_id": null,
        "count": 2,
        "students": [
            [
                "a",
            ],
        ]
    }
]

I tried several combinations of use setIntersection but any of that worked.

How can get only that intersection?

Thanks

CodePudding user response：

The multiple arrays intersection concept is similar to chridam's answer on How to find set intersection of sets between the documents in a single collection in MongoDB?.

1. $unwind: Deconstruct students array field to output documents for each element.
2. group: Group multiple documents to get initialStudent ($first).
3. $set: Create new field common. With $reduce operator to flatten the array for $setIntersection start from initialStudent and every next element in $students.
4. $project: Display student field with a nested array for the result of common in (3).

db.collection.aggregate([
  {
    "$unwind": "$students"
  },
  {
    "$group": {
      "_id": 0,
      count: {
        $sum: 1
      },
      "students": {
        "$push": "$students"
      },
      "initialStudent": {
        "$first": "$students"
      }
    }
  },
  {
    "$set": {
      "common": {
        "$reduce": {
          "input": "$students",
          "initialValue": "$initialStudent",
          "in": {
            "$setIntersection": [
              "$$value",
              "$$this"
            ]
          }
        }
      },
      
    }
  },
  {
    $project: {
      "count": 1,
      "students": [
        "$common"
      ]
    }
  }
])

Sample Mongo Playground

Improvement

Appreciate and credit @varman's suggestion on the query,

Pipelines 1 ($unwind) & 2 ($group) for the above solution can be simplified into $set pipeline to retrieve $first and $count for students.

{
  $addFields: {
    "initialStudent": {
      "$first": "$students"
    },
    "count": {
      "$size": "$students"
    }
  }
}

CodePudding user response：

One approach could be to do the intersection after getting the data.

//assuming students has only two arrays.
const IntersectedArray = students[0].filter(value => students[1].includes(value));