Rails: Global instance variables in Rails

Question

In my app users can submit recipes through a form, which will be published on a website. Before recipes get published they are moderated through a moderator.

Therefore my app shows in the navbar a count of all currently unpublished recipes for the moderator like so:

To achieve this at the moment I do the following:

application.rb

before_action :count_unpublished

def count_unpublished
  @unpublished_count = Recipe.where(:published => false).count
end

_navbar.html.erb

<li>
  <%= link_to recipes_path do %>
     Recipes <span class="badge" style="background-color:#ff7373"><%= @unpublished_count %></span>
  <% end %>
</li>

It works, but I am wondering now if this is a good practice as now with every action my app hits the recipe database (which is maybe not very elegant).

Is there a better solution to achieve this?

Abdullah · Accepted Answer · 2017-12-21 17:35:45Z

4

cache_key = "#{current_user.id}_#{unpublished_count}"
@unpublished_count = Rails.cache.fetch(cache_key, expires_in: 12.hours) do 
  Recipe.where(:published => false).count
end

For More: http://guides.rubyonrails.org/caching_with_rails.html#low-level-caching

edited Dec 21, 2017 at 17:35

answered Dec 21, 2017 at 16:54

Abdullah

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Sergio Tulentsev · Accepted Answer · 2017-12-21 16:50:52Z

3

To avoid hitting the database, you can introduce caching. It comes in many forms: faster storage (memcached, redis), in-process caching (global/class variables) and so on. And they all share the same problem: you need to know when to invalidate the cache.

Take a look at this guide to get some ideas: Caching with Rails.

If I were you, I would not care about this until my profiler tells me it's a performance problem. Instead, I'd direct my efforts to developing the rest of functionality.

answered Dec 21, 2017 at 16:50

Sergio Tulentsev

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3 Comments

engineersmnky Over a year ago

Hahaha I was just going to post this as a comment but apparently just "Caching" does not qualify as a legitimate comment

Sergio Tulentsev Over a year ago

@engineersmnky: same! had to add some fluff :)

MrYoshiji Over a year ago

To extend @SergioTulentsev answer, if this count becomes a performance problem, you can setup caching and ensure to invalidate the cache every time the Recipe record should not be counted anymore (ex: published changed (false => true or the reverse), Recipe deleted). This can be done with ActiveRecord callbacks (after_save, after_destroy).

Alexis · Accepted Answer · 2017-12-22 00:46:00Z

Your falling into the trap of premature optimisation. Before doing any optimisation (which increases code complexity most of the time) you have to profile your code to find the bottleneck. Improving a SQL requests which counts for a small part of the total response time is useless. In the contrary if the SQL takes a big amount of time, that is a great improvement.

For that I can recommend these 2 gems:

https://github.com/miniProfiler/rack-mini-profiler
https://github.com/BaseSecrete/rorvswild (disclaimer: I'm the author of this one)

To reply to your question, the better way would be:

# app/models/recipe.rb
class Recipe < AR::base
  # A nice scope that you can reuse anywhere
  scope :unpublished, -> { where(published: false) }
end

Then in your navbar.html.erb:

<li>
  <%= link_to recipes_path do %>
     Recipes <span class="badge" style="background-color:#ff7373"><%= Recipe.unpublished.count %></span>
  <% end %>
</li>

You have no more these ugly callback and instance variable in the controller.

Unless you have a lot of recipes (something like 100K or more) performance won't be an issue. In that case you can add an index:

CREATE INDEX index_recipes_unpblished ON recipes(published) WHERE published='f'

Note that the index applies only when published is false. Otherwise it would be counter productive.

I think that caching in your case is not well because the invalidation is extremely complex and leads to awful and easy breakable code. Don't worry to hit the database, we will never write faster code than PostgreSQL/MySQL, etc.

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