How to get reactive data of array intersection in Meteor?

Question

Let's say I have a collection of users and each user have an array with pages he likes (imagine facebook pages ids):

Current user:

{
  name: "me",
  likes: [123,234,777]
}

Target list:

{
  name: "Jane",
  likes: [111,222,333]
},
{
  name: "Mary",
  likes: [567,234,777,654]
},
{
  name: "John",
  likes: [123,234,567,890,777]
},
{
  name: "Steve",
  likes: [666,777,321,432,543]
},

Result:

{
  name: "John",
  likes: [123,234,777]
},
{
  name: "Mary",
  likes: [234,777]
},
{
  name: "Steve",
  likes: [777]
},
{
  name: "Jane",
  likes: []
},

I want to get an reactive publication of users ordered by users with whom I have the most common likes. I need'it reactive because each user have user presence - online/offline. The array with likes id's is static but user presence is changing over time.

Have a look at [this post](https://www.discovermeteor.com/blog/reactive-joins-in-meteor/) on reactive joins. — David Weldon, May 20 '14 at 18:38

score 1 · Accepted Answer · edited May 23 '17 at 11:45

I'll leave the publish/subscribe and reactive part to you, but the general mechanics of doing a set-intersection without having to iterate the results in code can be peformed using the aggregation framework.

Notably, this is a server side function only, and you need to obtain a reference to the underlying "node native driver" in order to invoke the .aggregate() method at present:

var db = MongoInternals.defaultRemoteCollectionDriver().mongo.db;

In fact for MongoDB 2.6 and above there is a $setIntersection operator that is available to make this simple:

db.targets.aggregate([

    // You probably want a $match to find "friends" first
    { "$match": { "_id": { "$in": [ friends ] } } },

    // Project the set intersection
    { "$project": {
        "name": 1,
        "likes": {
            "$setIntersection": [ "$likes", [123,234,777] ]
        }
    }}
])

It is still basically possible with MongoDB versions prior to 2.6, you just need to manually do all the work that the $setIntersection operator allows:

db.targets.aggregate([

    // You probably want a $match to find "friends" first
    { "$match": { "_id": { "$in": [ friends ] } } },

    // Project an array with the matching values
    { "$project": {
        "name": 1,
        "likes": 1,
        "mylikes": { "$cond": [ 1, [123,234,777], 0 ] }
    }},

    // Unwind both of the arrays
    { "$unwind": "$likes" },
    { "$unwind": "$mylikes" },

    // Work out where those likes are the same. Intersection.
    { "$project": {
        "name": 1,
        "likes": 1,
        "match": { "$eq": [ "$likes", "$mylikes" ] }  
    }},

    // Filter the results that did not match
    { "$match": { "match": true } },

    // Group back the array of likes
    { "$group": {
        "_id": "$_id",
        "name": { "$first": "$name" },
        "likes": { "$push": "$likes" }
    }}
])

That last one removes the entry for "Jane" since there are no matching likes, but is more or less the desired result.

If that interests you, then you should take a look at this post which covers using aggregate with meteor in more detail. But the general method is here and that should provide a snappier and more elegant solution than processing the results in the client in order to find the intersection of the sets.

How to get reactive data of array intersection in Meteor?

1 Answers1