session aware model methods in to_json cause n+1 queries - ruby-on-rails

Question

This is an optimization question for an existing application, I've made the code generic to both make it annonmous and also easier to understand, instead of our proprietary models I'm describing a Forum discussion type situation. I've modified all this code for this example and not tested it, so if there are any typos I apologize, I'll try to fix them if they are pointed out to me.
Lets say I have a rails app with four models: Event, User, Forum, and Post.
the important relationships are as follows:
User has many events.
Forum has many posts.
Post has many events.
The front end is a single page javascript app, so all database data needs to be returned in json format.
Context:
when a User clicks on a post, an event is created with the name
'Show' which marks the post as no longer new.
The user needs to be
logged in to see which posts are new clicking on a forum calls the
following endpoint:
There are multiple users so the events able is a many to many relationship between posts and users.
example.com/forum/15/all_posts
heres the relevant code:
Forum Controller:
#forums_controller.rb
def all_posts
current_user = User.find(session[:user_id])
forum = Forum.includes(:posts).where(id: params[:id]).take
forum.posts.each do |post|
post.current_user = current_user
end
render json: forum.to_json(
include: [
{ posts: {
methods: [:is_new]
}}
]
)
end
Posts model:
#post.rb (posts model)
has_many :events
attr_accessor :current_user
def is_new
if current_user #user may not be logged in
!!self.events.where(user_id: current_user.id, name: 'Show').take
else
false
end
end
the model is where the action is at, so we've tried to keep logic out of the controller, but since the session is not available in the model we end up with this crazy work around of adding current_user as an attr_accessor so that methods can return the appropriate data for the user in question.... I don't like this but I've never come up with a better way to do it. We've repeated this pattern elsewhere and I would love to hear alternatives.
Here's my problem:
The call to is_new is used on the front end to determine what posts to hi-light but it's also triggering an n+1 scenario If there are 10 posts, this endpoint would net me a total 12 queries which is no good if my events table is huge. If I moved all the logic to the controller I could probably do this in 2 queries.
in short I have two questions:
MOST IMPORTANT: How can I fix this n+1 situation?
Is there a better way in general? I don't like needing an each loop before calling to_json I don't find this pattern to be elegant or easy to understand. at the same time I don't want to move all the code into the controller. What is the rails way to do this?

Answer 1

If working with scope is an option, I will try something like:
class Post < ApplicationRecord
scope :is_new, -> { where(user_id: current_user.id, name: 'Show') } if current_user.id?
end
If is a better option to send the current_user in your case, you can also do it:
class Post < ApplicationRecord
scope :is_new, ->(current_user) {...}
end

Answer 2

This is just pseudo-code to give an example:
First Answer
When I posted this I forgot you are rendering json from ForumsController.
Post
scope :for_user, -> (user = nil) do
includes(events: :users).where(users: {id: user.id}) if user
end
def is_new_for_user?(user = nil)
return true if user.nil?
self.events.empty?{ |e| e.name == 'Show' }
end
PostController
def index
#posts = Post.for_user(current_user)
end
posts/index.html.erb
...
<% if post.is_new_for_user?(current_user) %>
...
<% end
...
Second Answer
This is still pseudo-code. I didn't test anything.
Forum
scope :for_user, -> (user = nil) do
if user
includes(posts: [events: :users]).where(users: {id: user.id})
else
includes(:posts)
end
end
ForumsController
def all_posts
current_user = User.find(session[:user_id])
forum = Forum.for_user(current_user).where(id: params[:id]).take
render json: forum.to_json(
include: [
{ posts: {
methods: [:is_new_for_user?(current_user)]
}}
]
)
end