I made this plugin when we migrated to Rails 3. It basically prevents the development team from triggering any database query from the views.

TL;DR Make sure you understand the data you need in your views, because manipulating directly ActiveRecord objects in your view is both powerful (follow associations, use model methods, etc.) and dangerous (N+1 problem). (and yes, this is a copy of the No Querying Views README because the blog was not broken at the time I published it)

This Rails3 plugin will tell you when you and your folks are querying the database from something else than a controller and a model.

It supports SQLite, SQLite3, MySQL, PostgreSQL and Mongoid.

Context

Rails is a MVC framework. The separation of concern should be respected to prevent bad things to happen in your application.

Example: An order has N order lines. Each of these order lines has 1 related item, and each of these items belongs to 1 category.

In Ruby code, your models would look like this:

app/models/order.rb
class Order < ActiveRecord::Base
  has_many :lines
end
app/models/line.rb
class Line < ActiveRecord::Base
  has_one :item
end
app/models/item.rb
class Item < ActiveRecord::Base
  belongs_to :category
end
app/models/category.rb
class Category < ActiveRecord::Base
end

Now, if you want to display them on a single page, here is a naive version of order_controller.rb:

app/controllers/order_controller.rb
class OrderController < ApplicationController
  def show
    @order = Order.find(params[:id])  # 1 query
  end
end

And a naive version of the views used to display the result:

app/views/orders/show.html.haml
%h1
  = @order.title
%h2
  %span Items
  %ul
    = render :partial => 'lines', :collection => order.lines   # 1 query, so far: 2 queries
app/views/orders/_lines.html.haml executed for each single line
%li
  %span
    = line.item.title          # 1 query
    = line.price
  %div{:class => 'item_description'}
    = line.item.description    # 0 query (cached)
    = line.item.category       # 1 query

The problem

If you use a tool that generates execution traces (like NewRelic RPM), you will see that - for 1 order, 40 lines, 30 items, 20 categories - you will execute: 1 (select * from order) + 1 (select * from lines inner join … where order_id = foo) + 40 (select * from items inner join … where line.item_id = bar) + 40 (select * from categories inner join … where item.category_id = baz) —- 82 queries against the database!

This is called the N + 1 problem and it can be solved by an eager-loading strategy.

## OH: “Yeah, but my database is so small and so fast that I don’t even see that!”

This is true if you are the only user of the website. But Rails will also have troubles generating the page (HAML, ERB, whatever) and NewRelic won’t tell you that the queries are slowing down the application: the queries are fast as hell.

The combination of the rendering and the querying is slowing down the application, because:

  1. Rendering a partial is slower than inline code
  2. Executing a query costs time (retrieving the DB connection within the pool, executing and fetching the results, …): the resources are not being used for free on your machine.

The solution

Silver bullets don’t exist, but most of the time you can speed up the performance by eager-loading the data you expect in your views.

From an architectural point of view, it is wrong to query the DB from a view. There are many reasons not to do so and the internet is full of literature about it.

Right now, here is a piece of code to help you remove any DB query from your views.

What does this code do?

It will raise an exception everytime you and your people try to query the database from within a view.

It only overrides the execute method of the adapter and checks the call stack.

Rails 3.x

ActiveRecord

Install
cd /my/rails/3/app_root/
wget http://github.com/franckverrot/no_querying_views/raw/master/no_querying_views.rb > config/initializers/no_querying_views.rb
Uninstall
cd /my/rails/3/app_root/
rm config/initializers/no_querying_views.rb

Mongoid

Install
cd /my/rails/3/app_root/
wget http://github.com/franckverrot/no_querying_views/raw/master/mongoid_no_querying_views.rb > config/initializers/mongoid_no_querying_views.rb
Uninstall
cd /my/rails/3/app_root/
rm config/initializers/no_querying_views.rb