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:

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/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:

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person_path(@person)

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

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

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person_path(@person.id) # Bad!

Instead, always pass the object:

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person_path(@person)

Slug Generation with to_param

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

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

The parameterize method from ActiveSupport will turn any string into characters valid for in a URL.

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

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/people/6-bob-martin

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:

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"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.

Another limitation is that the numeric ID is still in the URL. If the ID is something you want to obfuscate, simple slug generation by overriding to_param 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 lookups. Imagine we have a Tag object that has a name column. The name would be something like ruby or rails.

Link Generation

We can again override to_param for creating the links:

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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 path 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] by the router.

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 use Tag.find_by_name(params[:id]). It will work, but it is 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:

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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:

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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:

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

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

And there is a better way, based on these two facts:

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

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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:

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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 have introduced a possible bug: if a name starts with a digit it will look like an ID. Let’s add a validation that names cannot start with a digit:

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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? If so, it may be worth checking out the FriendlyID gem: https://github.com/norman/friendly_id

Setup

Add the gem to your Gemfile:

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gem "friendly_id", "~> 4.0.0"

Then run bundle from the command line.

Simple Usage

The minimum configuration required in your model is:

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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

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 every request. 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:

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

Usage

You can see it in action here:

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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.

Exercises

1. Implement a to_param method in Article so URLs include the id and article title like 4-hello-world
2. Change the to_param in Article so the output does not include the id
3. Try to modify the show action of ArticlesController so the lookup with work with the no-id-having slug from exercise 2. Why is this impossible to implement efficiently?
4. Implement FriendlyID, as described above, so tags use only their name in URLs.
5. Implement FriendlyID so article URLs no longer use the id, only the article’s parameterized title.