Friendly URL - Jiazhen Xie's blog

This is a great article Friendly URLs. So that I repost here.

By default, Rails applications build URLs based on the primary key — the id column from the database. Imagine we have a Person model and associated controller. We have a person record for Bob Martin that has id number 6. The URL for his show page would be:

/people/6

But, for aesthetic or SEO purposes, we want Bob’s name in the URL. The last segment, the 6 here, is called the “slug”. Let’s look at a few ways to implement better slugs.

Simple Approach

The simplest approach is to override the to_param method in the Person model. Whenever we call a route helper like this:

person_path(@person)

Rails will call to_param to convert the object to a slug for the URL. If your model does not define this method then it will use the implementation in ActiveRecord::Base which just returns the id.

For this method to succeed, it’s critical that all links use the ActiveRecord object rather than calling id. Don’t ever do this:

person_path(@person.id) # Bad!

Instead, always pass the object:

person_path(@person)

Slug Generation

Instead, in the model, we can override to_param to include a parameterized version of the person’s name:

class Person < ActiveRecord::Base
  def to_param
    [id, name.parameterize].join("-")
  end
end

For our user Bob Martin with id number 6, this will generate a slug 6-bob_martin. The full URL would be:

/people/6-bob-martin

The parameterize method from ActiveSupport will deal with converting any characters that aren’t valid for a URL.

Object Lookup

What do we need to change about our finders? Nothing! When we call Person.find(x), the parameter x is converted to an integer to perform the SQL lookup. Check out how to_i deals with strings which have a mix of letters and numbers:

> "1".to_i
# => 1 
> "1-with-words".to_i
# => 1 
> "1-2345".to_i
# => 1 
> "6-bob-martin".to_i
# => 6

The to_i method will stop interpreting the string as soon as it hits a non-digit. Since our implementation of to_param always has the id at the front followed by a hyphen, it will always do lookups based on just the id and discard the rest of the slug.

Benefits / Limitations

We’ve added content to the slug which will improve SEO and make our URLs more readable.

One limitation is that the users cannot manipulate the URL in any meaningful way. Knowing the url 6-bob-martin doesn’t allow you to guess the url 7-russ-olsen, you still need to know the ID.

And the numeric ID is still in the URL. If this is something you want to obfuscate, then the simple scheme doesn’t help.

Using a Non-ID Field

Sometimes you want to get away from the ID all together and use another attribute in the database for lookup. Imagine we have a Tag object that has a name column. The name would be something like ruby or rails.

Link Generation

Creating links can again override to_param:

class Tag < ActiveRecord::Base
  validates_uniqueness_of :name
  
  def to_param
    name
  end
end

Now when we call tag_path(@tag) we’d get a URL like /tags/ruby.

Object Lookup

The lookup is harder, though. When a request comes in to /tags/ruby the ruby will be stored in params[:id]. A typical controller will call Tag.find(params[:id]), essentially Tag.find("ruby"), and it will fail.

Option 1: Query Name from Controller

Instead, we can modify the controller to Tag.find_by_name(params[:id]). It will work, but it’s bad object-oriented design. We’re breaking the encapsulation of the Tag class.

The DRY Principle says that a piece of knowledge should have a single representation in a system. In this implementation of tags, the idea of “A tag can be found by its name” has now been represented in the to_param of the model and the controller lookup. That’s a maintenance headache.

Option 2: Custom Finder

In our model we could define a custom finder:

class Tag < ActiveRecord::Base
  validates_uniqueness_of :name
  
  def to_param
    name
  end
  
  def self.find_by_param(input)
    find_by_name(input)
  end
end

Then in the controller call Tag.find_by_param(params[:id]). This layer of abstraction means that only the model knows exactly how a Tag is converted to and from a parameter. The encapsulation is restored.

But we have to remember to use Tag.find_by_param instead of Tag.find everywhere. Especially if you’re retrofitting the friendly ID onto an existing system, this can be a significant effort.

Option 3: Overriding Find

Instead of implementing the custom finder, we could override the find method:

class Tag < ActiveRecord::Base
  #...
  def self.find(input)
    find_by_name(input)
  end
end

It will work when you pass in a name slug, but will break when a numeric ID is passed in. How could we handle both?

The first temptation is to do some type switching:

class Tag < ActiveRecord::Base
  #...
  def self.find(input)
    if input.is_a?(Integer)
      super
    else
      find_by_name(input)
    end
  end
end

That’ll work, but checking type is very against the Ruby ethos. Writing is_a? should always make you ask “Is there a better way?”

Yes, based on these facts:

Databases give the id of 1 to the first record
Ruby converts strings starting with a letter to 0

class Tag < ActiveRecord::Base
  #...
  def self.find(input)
    if input.to_i != 0
      super
    else
      find_by_name(input)
    end
  end
end

Or, condensed down with a ternary:

class Tag < ActiveRecord::Base
  #...
  def self.find(input)
    input.to_i == 0 ? find_by_name(input) : super
  end
end

Our goal is achieved, but we’ve introduced a possible bug: if a name starts with a digit it will look like an ID. If it’s acceptable to our business domain, we can add a validation that names cannot start with a digit:

class Tag < ActiveRecord::Base
  #...
  validates_format_of :name, :without => /^d/
  def self.find(input)
    input.to_i == 0 ? find_by_name(input) : super
  end
end

Now everything should work great!

Using the FriendlyID Gem

Does implementing two additional methods seem like a pain? Or, more seriously, are you going to implement this kind of functionality in multiple models of your application? Then it might be worth checking out the FriendlyID gem: https://github.com/norman/friendly_id

Setup

The gem is just about to hit a 4.0 version. As of this writing, you want to use the beta. In your Gemfile:

gem "friendly_id", "~> 4.0.0.beta8"

Then run bundle from the command line.

Simple Usage

The minimum configuration in your model is:

class Tag < ActiveRecord::Base
  extend FriendlyId
  friendly_id :name
end

This will allow you to use the name column or the id for lookups using find, just like we did before.

Dedicated Slug

But the library does a great job of maintaining a dedicated slug column for you. If we were dealing with articles, for instance, we don’t want to generate the slug over and over. More importantly, we’ll want to store the slug in the database to be queried directly.

The library defaults to a String column named slug. If you have that column, you can use the :slugged option to automatically generate and store the slug:

class Tag < ActiveRecord::Base
  extend FriendlyId
  friendly_id :name, :use => :slugged
end

Usage

You can see it in action here:

> t = Tag.create(:name => "Ruby on Rails")
# => #<Tag id: 16, name: "Ruby on Rails", created_at: "2011-09-11 15:42:53", updated_at: "2011-09-11 15:42:53", slug: "ruby-on-rails"> 
> Tag.find 16
# => #<Tag id: 16, name: "Ruby on Rails", created_at: "2011-09-11 15:42:53", updated_at: "2011-09-11 15:42:53", slug: "ruby-on-rails"> 
> Tag.find "ruby-on-rails"
# => #<Tag id: 16, name: "Ruby on Rails", created_at: "2011-09-11 15:42:53", updated_at: "2011-09-11 15:42:53", slug: "ruby-on-rails"> 
> t.to_param
# => "ruby-on-rails"

We can use .find with an ID or the slug transparently. When the object is converted to a parameter for links, we’ll get the slug with no ID number. We get good encapsulation, easy usage, improved SEO and easy to read URLs.

If you’re adding FriendlyId to an existing app and need to generate slugs for existing users, do this from the console, runner, or add a Rake task User.find_each(&:save)