Love Your lib Directory


CEO & Founder

Brian Cardarella

Be sure to check out Bryan Helmkamp’s blog post on the same topic

The lib/ directory is the Red Headed Stepchild of your Rails application. Let’s discuss some conventions for keeping it clean and what should and shouldn’t go in there.

It’s not a dump


Does this look familiar? It does to me. This is what my lib/ directory looked like before I got fed up with it. That truck, that was me dumping more code into lib/.

In my experience there is one outstanding reason why code ends up getting dumped into the lib/ directory: A poor understanding of what a model is. Rails has this way of reinforcing bad habits. Perhaps because it is so easy to get going some developers never bother to learn that a model does not in any way need to be attached to a persitence layer. (i.e. ActiveRecord)

Let’s all agree to the following:

  1. All Business Logic Goes Into A Model
  2. All Models Go Into app/models

When we say “Business Logic” we are of course talking about “Application Specific Business Logic”. There is always the case of something you’re working on that is so generic it can be shared with other applications you are (or will be) working on. Or, even better, with the community in general as Open Source. That brings me to the next point.

Understanding the load path

If you have written a Rubygem, or at the very least, looked through one, you know that the lib/ directory is special. The short version of the story is that Rubygems iterates over all of the libraries you have installed as a gem, and appends any lib/ directories onto Ruby’s Load Path. This is basically how Ruby gem files are exposed, so when you as do a gem require it will iterate through every path in the load path and give you the first match.

This is also true with Rails. After all of your gems are loaded and your application is up Rails will append ./lib/ to your load path. Any files you put in there can now be required the exact same way gems are. This gives us an excellent path to extracting general functionality out into. You can even play tricks with this, in your application.rb file put the following at the top:

$:.unshift(File.expand_path('../../lib', __FILE__))

Now in your lib directory create an ‘active_record’ directory and add a file called ‘base.rb’. Inside that file add the following:


Load up your Rails app and watch it throw an exception. Why? Because your app’s lib/ directory was prepended to the load paths and when the lookup for active_record/base happened the first match was in your app’s lib/ instead of in the proper gem. This of course is more of an interesting hack than anything really useful. But it does do a good job of demonstrating how Rubygems’ lookup happens.

Use initializers for initializing, that is all

I have seen developers dump code into initializers that has no business being there. Yes, it loads and it works. That is not the point. We have conventions for a reason. Any code that you feel needs to go into an initializer and has nothing to do with actually setting preferences or something of that manner almost always should go into the lib/ directory. If you must monkey patch. Put it into the lib/ directory. If you are creating a new class or module that has no business being in app/models put it in to the lib/ directory.

Using lib/ to extend core, stlib, or a gem

Far too often I’ve needed to extend a class that is being defined outside of my project. There are a few ways to deal with this. You can use a Composite to define a new class that you can then play around with. The downside to this is that I sometimes want to modify a class that is being inherited by other classes. This is when I think it is appropriate to Monkey Patch.

The pattern I have fallen upon is to define a gem_ext/ directory and a gem_ext.rb file in lib. I then make sure the extensions are loaded up using an initializer. For lack of a better term I call this lib_loader.rb. Lets start with the loader.

# config/initializers/lib_loader.rb

require 'gem_ext'

Simple enough. Now for this example I’ll use a Haml custom filter I wrote. This filter allows me to write Handlebars templates in my views like so:

-# app/views/home/show.html.haml

  // handlebars code goes here

Now I can easily add handlebar templates to any haml file. This is how I did it.

Under lib/gem_ext I defined a haml/ directory and a haml.rb file. Then I defined haml/custom_filters.rb and inside that file I added

# lib/gem_ext/haml/custom_filters.rb

module Haml::Filters
  module Handlebars
    include Base

    def render_with_options(text, options)
      type = " type=#{options[:attr_wrapper]}text/x-handlebars#{options[:attr_wrapper]}"
  #{text.rstrip.gsub("\n", "\n    ")}

Now in haml.rb I added

# lib/gem_ext/haml.rb

require 'gem_ext/haml/custom_filters'

And finally in gem_ext.rb I added

# lib/gem_ext.rb

require 'gem_ext/haml'

This gives me a very clean approach to extending classes without worrying about muddying up the load path with name collisions or other surprises. In addition this pattern can be repeated for Core and Stdlib classes in core_ext and stdlib_ext respectively.

Using lib/ as a pattern to extracting Rubygems

A pattern I have fallen upon when wanting to extract functionality out of an app into a Rubygem has been to first extract that code into the lib/ directoy. From there I have a nice way to test the code in isolation. I am also forced to write the code as a class independent from my app. After I am satisfied with what I have I can think about extracting that into an external gem.

A great example of this is something that Patrick Robertson wrote for BostonRB

We wanted to show the next upcoming event at the top of the website. All of our events are stored in a Google Calendar. Unfortunately most of the Google Calendar gems out there are crap. Patrick decided to roll his own.

You can see that the boston_rb_calendar.rb is requiring several files just like any Gem would. Because of the isolation he was able to test the class very easily.

From here, if Patrick wanted to release this as a gem it wouldn’t take too much effort. Some renaming of classes would be required but he has all of the major parts in place.

Go forth and show some <3<3<3<3

Keeping your code clean pays itself forward in many way. The team you are apart of or the team you are handing off to will thank you. Heck, your future self might thank you. The patterns I’ve described here are ones that I have found success with. If you have noticed other patterns concerning the lib/ directory please feel free to comment!

Narwin holding a press release sheet while opening the DockYard brand kit box