how to find most mutual friend or follower?

Question

I have a list of friends and those friends have a list of their own friends and I need to calculate the most mutual friend for me. I am using MongoDB as a database.

my user schema

    id: {
    type: String,
    require: true,
    unique: true
},
    followers: [{
        type:mongoose.Schema.ObjectId,
        ref: 'User'
    }],
    following: [{
        type:mongoose.Schema.ObjectId,
        ref: 'User'
    }]
})

I am not sure but is it possible to achieve this feature using aggregation in MongoDB.

Answer 1

You can use this aggregation query.

1. Find both users based on _id. ( this should return 2 documents).

2. Group both the documents using $first and $last. ( this will combine both documents).

3. Use Project stage to find the count ($setIntersection will give you mutual elements of the array, then you can compute the size).

   [{
   '$match': {
       '_id': {
           '$in': [
               ObjectId('60d90d5163326a4760b68ee3'), ObjectId('60d90d5dc498a4477a039d28')
           ]
       }
    }
   }, {
   '$group': {
       '_id': 0, 
       'set1': {
           '$first': '$following'
       }, 
       'set2': {
           '$last': '$following'
       }
    }
   }, {
   '$project': {
       'mutual': {
           '$setIntersection': [
               '$set1', '$set2'
           ]
       }, 
       'count': {
           '$size': {
               '$setIntersection': [
                   '$set1', '$set2'
               ]
           }
       }
    }
   }]