CODEWITHSUNDEEP
arraysmongodbmongodb-queryaggregation-framework
David S.
David S.

Reputation: 11168

How to get the intersection of two arrays in MongoDB ($setIntersection does not work well)

I have a simple data structure that defines people and their friends.

{
  id: 0,
  name: 'a',
  friends: [1,2,3]
}

I need to find the common friends of two people. I managed to use the aggregation pipleline get the friends array into an array.

{ "_id" : 0, "friends" : [ [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 ], [ 0, 1, 2 ] ] }

So the friends field is a nested array, and I want to get the intersection of its elements.

I tried to use the $setIntersection operation, however I found it does not accept a variable of an array, it only accepts an array of variables. So I have to use something like this to get the result.

{
  $project: {
    commonFriendIds: {
      $setIntersection: [
        { $arrayElemAt: ["$friends", 0] },
        { $arrayElemAt: ["$friends", 1] }
      ]
    }
  }
}

It looks ugly, and I have to modify the code to if I need to get the common friends of 3 or more people.

Is there's a better way to accomplish this?

Upvotes: 2

Views: 967

Answers (1)

Ashh
Ashh

Reputation: 46441

You can use $reduce aggregation operator. Now It will provide you the intersection for all the nested arrays inside the friends array.

db.collection.aggregate([
  { "$project": {
    "commonFriendIds": {
      "$reduce": {
        "input": "$friends",
        "initialValue": { "$arrayElemAt": ["$friends", 0] },
        "in": { "$setIntersection": ["$$this", "$$value"] }
      }
    }
  }}
])

MongoPlayground

Upvotes: 4

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