Slugging logic out of the model.

migration.rb

# For when you're adding the slug column to a model with existing records:
SlugGenerator.slug User, :slug

model.rb

# Need a slug to be there, be unique, and follow an appropriate format.
validates :slug, presence: true, uniqueness: true,
  format: {
    with: /\A[a-z0-9\-]+\z/,
    message: 'must be lowercase letters, numbers and dashes only'
  }

# Generate slug by the name column
before_validation SlugCallback.new(:slug, [:name]), on: :create
# Update the slug if the name changes
before_validation SlugCallback.new(:slug, [:name]), on: :update, if: :name_changed?

slug_callback.rb

class SlugCallback
  attr_reader :column, :parts

  def initialize(column, parts = [:name])
    @column, @parts = column, parts
  end

  def before_validation(instance)
    SlugGenerator.new(column, parts, instance).call
  end
end

slug_generator.rb

class SlugGenerator
  CLEAN_REGEX_PAIRS = [[/[\s_]+/,  '-'], [/[^\w\-]/, ''], [/\-+/, '-']]

  def self.slug(klass, column, parts = [:name])
    klass.where(column => nil).each do |instance|
      new(column, parts, instance).call
      instance.save!
    end
  end

  def initialize(column, parts, instance)
    @column, @parts, @instance = column, parts, instance
  end

  def call
    instance.send "#{column}=", unique_slug
  end

  private

  attr_reader :column, :parts, :instance

  def generated_slug
    slug_parts.compact.collect { |part| sluggify part }.join('-')
  end

  def sluggify(string)
    CLEAN_REGEX_PAIRS.inject(string.downcase) do |string, pair|
      string.gsub pair.first, pair.last
    end
  end

  def slug_available?(slug)
    existing = instance.class.where(column => slug).first
    existing.nil? || existing.id == instance.id
  end

  def slug_parts
    parts.collect { |part| instance.send part }
  end

  def unique_slug
    slug = generated_slug
    return slug if slug_available?(slug)

    index  = 1
    index += 1 until slug_available?("#{slug}-#{index}")
    "#{slug}-#{index}"
  end
end