Default Scoping

It’s pretty common to want SQL queries against a particular table to always be sorted the same way, and is one of the reasons why I added the ordered scope to the utility scopes gem. For instance, when dealing with collections of articles it is reasonable to expect that the default ordering be most recent first, i.e. created_at DESC. Well now you can specify default ordering, and other scopes, in edge rails directly in your ActiveRecord model.

Taking our Article example let’s specify the aforementioned default ordering:

class Article < ActiveRecord::Base
  default_scope :order => 'created_at DESC'
end

Now, when any find method or named_scope is executed the default ordering comes along for the ride:

Article.find(:all) #=> "SELECT * FROM `articles` ORDER BY created_at DESC"

The same holds true for any named scopes you might have:

class Article < ActiveRecord::Base
  default_scope :order => 'created_at DESC'
  named_scope :published, :conditions => { :published => true }
end

Article.published #=> "SELECT * FROM `articles` WHERE published = true ORDER BY created_at DESC"

There are some things to keep in mind, however. First is that scopes like :join, :offset, :limit and :order will get clobbered by the innermost rule. For example, here the default scope ordering loses out to the named_scope ordering.

class Article < ActiveRecord::Base
  default_scope :order => 'created_at DESC'
  named_scope :published, :conditions => { :published => true },
                          :order => 'published_at DESC'
end

# published_at DESC clobbers default scope
Article.published #=> "SELECT * FROM `articles` WHERE published = true
ORDER BY published_at DESC"

Also keep in mind that the default scoping is inherited, so child-classes of Article will have the same default scoping.

And for those occasions when you want to override or remove your default scope, just use with_exclusive_scope:

class Article < ActiveRecord::Base
  default_scope :order => 'created_at DESC'
end

# Ignore other scoping within this block
Article.with_exclusive_scope { find(:all) }  #=> "SELECT * FROM `articles`

default_scope is a great way to specify reasonable query defaults and relieve yourself of having to create your own named_scopes for that purpose or specify such conditions at every invocation. Yes, I just said ‘relieve yourself’... giggity giggity

http://ryandaigle.com/articles/2008/11/18/what-s-new-in-edge-rails-default-scoping