watch_list.rb

class Watchlist
  # Will generate SELECT * FROM users WHERE id IN(SELECT user_id FROM watchlistings WHERE item_id = ...)).
  # Instead of two queries (one to pluck the ids, the other to get the watchlistings.
  def self.users_watching(item)
    ids = Watchlisting.where(item_id: item.id).select(:user_id)
    User.where(id: ids)
  end

  def initialize(user)
    @user = user
  end

  # Same thing here.
  def items
    ids = Watchlisting.where(user_id: @user.id).select(:item_id)
    Item.where(id: ids)
  end

  def add(item)
    Watchlisting.create!(user_id: @user.id, item_id: item.id)
    item
  end
end