CODEWITHSUNDEEP
ruby-on-railsoptimizationmongoideager-loading
Cyril Duchon-Doris
Cyril Duchon-Doris

Reputation: 14029

Mongoid eager loading with embedded documents

Some of my classes :

class User
  embeds_many :notifications
  field :first_name
  field :last_name

  def name{ "#{first_name} #{last_name}" }

class Notification
  embedded_in :user
  belongs_to :sender, class_name: "User", inverse_of: nil

Now in my views, I implemented a small mailbox system for notifications. However, it's currently hitting N+1 times the database :

<% current_user.notifications.sort{...}.each do |notif|%>
    ...
    <%= notif.sender.name if notif.sender %>

The problem here is the notif.sender.name which causes N hits on the database. Can I somehow preload/eager load this ? Something like current_user.notifications.includes(:sender) (but which would work :D)

I currently only need the sender name.

Upvotes: 3

Views: 3112

Answers (2)

Jos&#233; Coelho
Jos&#233; Coelho

Reputation: 526

It's not perfect, but this article presents a possible solution.

You can load all the senders and use set_relation to avoid them to be loaded every time.

def notifications_with_senders
  sender_ids = notifications.map(:sender_id)
  senders = User.in(id: sender_ids).index_by(&:id)
  notifications.each do |notification|
   notification.set_relation(:sender, senders[notification.sender_id])
  end
end

Would be great to have that as a Relation method (like includes of Rails Active Record)

Upvotes: 3

mu is too short
mu is too short

Reputation: 434805

I think you're half out of luck here. Mongoid has an error message like:

Eager loading in Mongoid only supports providing arguments to M.includes that are the names of relations on the M model, and only supports one level of eager loading. (ie, eager loading associations not on the M but one step away via another relation is not allowed).

Note the last parenthesized sentence in particular:

eager loading associations not on the M but one step away via another relation is not allowed

Embedding is a relation but you want to apply includes to the embedded relation and that's one step too far for Mongoid.

The fine manual does say that:

This will work for embedded relations that reference another collection via belongs_to as well.

but that means that you'd call includes on the embedded relation rather than what the models are embedded in. In your case, that means that you could eager load the senders for each set of embedded Notifications:

current_user.notifications.includes(:sender).sort { ... }

That still leaves you with the N+1 problem that eager loading is supposed to get around but your N will be smaller.

If that's still too heavy then you could denormalize the name into each embedded document (i.e. copy it rather than referencing it through the sender). Of course, you'd need to maintain the copies if people were allowed to change their names.

Upvotes: 3

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