Enable random access to collection with MongoDB

Question

I have a Rails application. It has a feed that shows items from different users all mixed up. It would be something similar to Pinterest in the way you see these items.

Right now I show all these items ordered by its date of creation. However, as the items are created by batches by users, they are shown not randomly (say you se the first 6 items being from one user, then the other 5 from other one, etc.).

The code that serves the items is this:

class Feeder

    def self.most_recent_created(watching_user=nil, current_cursor)

        next_cursor = nil
        feed = []
        influencers_ids = User.influencers.distinct(:_id)

        Rating.most_recent_from_influencers(watching_user, influencers_ids).scroll(current_cursor) do |rating, cursor|
            next_cursor = cursor
            feed << ImoPresenter.new(Imo.new(rating), watching_user)
        end

        feed << next_cursor.to_s

    end

end

scroll just gives a cursor pointing to each item in the iteration. Then I push the item into the feed.

The access to the database is done in Rating.most_recent_from_influencers(watching_user, influencers_ids), where most_recent_from_influencers(watching_user, influencers_ids) is a scope defined as follows:

scope :not_from, ->(user) { ne(user_id: user.id) }
scope :from, ->(user_ids) { any_in(user_id: user_ids) }
scope :most_recent_from_influencers, ->(watching_user, influencers_ids) {
        proxy = from(influencers_ids).over_zero.desc(:created_at).limit(IMOS_PER_PAGE)
        proxy = proxy.not_from(watching_user) if watching_user
        proxy
    }

MongoDB does not have random access out of the box. They suggest this for having a way of accessing randomly to the items. Basically, the solution is to add a random field in all documents and order the collection through this field. However, although I would have random items, I would always have almost the same items being shown, as I would just have the options of ordering it by desc(:rand) or asc(:rand) .

I would like to have suggestions on how I can make the items being shown truly in a random way. Is it possible?

Answer 1

Based on similar questions, I've come to the conclusion that having a random field is a valid solution if the collection is dynamic, meaning that documents are inserted frequently. The more dynamic the collection is, the more 'random' access you can have.