```ruby
# CMS web page. Tree structure. Versioned.
# User render_page helper to look for Page before falling back on Rails templates.
# Pages uses ERb and can execute Ruby code just like a template, so admin users can
# do things like <% Person.destroy_all %>!
class Page < ActiveRecord::Base
  acts_as_tree
  include ActsAsTree::Validation

  include Pages::Paths
  include RacingOnRails::VestalVersions::Versioned
  include SentientUser

  before_validation :set_slug, :set_path, :set_body
  validates_uniqueness_of :path, message: "'%{value}' has already been taken"

  after_create :update_parent
  after_destroy :update_parent

  # Friendly param. Last segment in +path+
  def set_slug
    self.slug = title.downcase.gsub(" ", "_") if slug.blank?
    slug
  end

  # Can't reliably set default value for MySQL text field
  def set_body
    self.body = "" unless body
    body
  end

  def update_parent
    if parent(true)
      parent.skip_version do
        parent.touch
      end
    end
    true
  end

  def valid_parents
    Page.all.to_a.delete_if { |page|
      page == self || descendants.include?(page)
    }
  end

  def depth
    ancestors.size
  end

  def to_s
    "#<Page #{id} #{title} #{slug} #{path}>"
  end
end
```

Rewrite valid_parents method a bit prettier
user
```ruby
# CMS web page. Tree structure. Versioned.
# User render_page helper to look for Page before falling back on Rails templates.
# Pages uses ERb and can execute Ruby code just like a template, so admin users can
# do things like <% Person.destroy_all %>!
class Page < ActiveRecord::Base
  acts_as_tree
  include ActsAsTree::Validation

  include Pages::Paths
  include RacingOnRails::VestalVersions::Versioned
  include SentientUser

  before_validation :set_slug, :set_path, :set_body
  validates_uniqueness_of :path, message: "'%{value}' has already been taken"

  after_create :update_parent
  after_destroy :update_parent

  # Friendly param. Last segment in +path+
  def set_slug
    self.slug = title.downcase.gsub(" ", "_") if slug.blank?
    slug
  end

  # Can't reliably set default value for MySQL text field
  def set_body
    self.body = "" unless body
    body
  end

  def update_parent
    if parent(true)
      parent.skip_version do
        parent.touch
      end
    end
    true
  end

  def valid_parents
    Page.all.select do |page|
      page != self && !descendants.include?(page)
    end
  end

  def depth
    ancestors.size
  end

  def to_s
    "#<Page #{id} #{title} #{slug} #{path}>"
  end
end
```
assistant