document.rb

class Document < ActiveRecord::Base
  after_save :update_permalink
  
  def to_param
    permalink.blank? ? id.to_s : permalink
  end
  
  private
  
  def update_permalink
    if permalink.blank? && !(title.blank? || isbn.blank?)
      update_attribute(:permalink, "#{id}-#{title} #{isbn}".parameterize)
    end
  end
end

document_spec.rb

# SPECS
# Note: I am using blueprint for my 'Document' model. (gem install machinist)

describe "permalink" do
  
  before :each do
    @document = Document.make :title => 'I want to be in the permalink', :isbn => '978-5-2054-6661-5'
    @expected_permalink = "#{@document.id}-i-want-to-be-in-the-permalink-9785205466615"
  end
  
  it "should set a permalink" do
    @document.permalink.should eql(@expected_permalink)
  end
  
  it "should not change indirectly" do
    @document.update_attribute(:title, 'Oh I changed my mind')
    @document.reload.permalink.should eql(@expected_permalink)
    @document.update_attribute(:isbn, Faker::Isbn.isbn_13)
    @document.reload.permalink.should eql(@expected_permalink)
  end
  
  it "should not set a permalink if one of the permalink components is blank" do
    @document = Document.make_unsaved(:title => '')
    @document.save(false)
    @document.permalink.should eql(nil)
    @document = Document.make_unsaved(:isbn => '')
    @document.save(false)
    @document.permalink.should eql(nil)
  end
  
  it "should use the permalink in params" do
    @document.to_param.should eql(@expected_permalink)
  end
  
  it "should use the id in params if no permalink is set" do
    @document.permalink = nil
    @document.to_param.should eql(@document.id.to_s)
  end
  
  it "should fetch a document the conventional way" do
    # Hint: Rails is converting the given id attribute to an integer. This works because '123-whatever'.to_i evaluates as 123.
    Document.find(@expected_permalink).should eql(@document)
    Document.find_by_id(@expected_permalink).should eql(@document)
  end
end

some bad words about friendly_id

F**K FRIENDLY ID. Why did I ever believe in you?
Yes, it was my very own fault to install this piece of s**t.

To all other people wondering if they should install a gem to take care of friendly ids. Note that I use the term 'permalink' in the meaning of a 'nice looking permalink' or 'Friendly Permalink'.

* It is a bad decision to go with a solution on steroids (or from texas) if you just need to accomplish one simple thing: Nice looking urls that don't break your existing code.

* Versioned permalinks? Way too nerdy plus bad idea at all. If you fail to create a useful permalink in the first place, you should not even start to create permalinks.
There are some hardcore 2 % edge cases often related to a legacy codebase. Anyways, projects with such issues scream for a rewrite.

* Get Real! Think about your business case. Acceptance tests have shown that friendly permalinks are a luxury problem. Do you _really_ need to solve a luxury problem?

* 99% of users just care about the usefulness of your application, not about good looking URI's. The kind of persons (sorted by stupidity) who care are:
  * Managers who think that the SEO guy is more worth than the developer
  * Overpayed SEO guys
  * Plain old Nitpicks
  * Managers that don't listen to their developers

* 'Friendly Permalinks' have one bad aspect: They should be permanent and should never change. It _will_ cost you a lot of resources (time, money) migrating permalinks from old to new! Therefore think twice as much before implementing such a luxury thing. Use the simplest most lasting solution (IDS MAYBE???)

Talk to me if you feel uncomfortable with my write up. @Overbryd