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method

named_scope

Importance_5
v2.2.1 - Show latest stable - 16 notes - Class: ActiveRecord::NamedScope::ClassMethods
named_scope(name, options = {}, &block) public

Adds a class method for retrieving and querying objects. A scope represents a narrowing of a database query, such as :conditions => {:color => :red}, :select => 'shirts.*', :include => :washing_instructions.

  class Shirt < ActiveRecord::Base
    named_scope :red, :conditions => {:color => 'red'}
    named_scope :dry_clean_only, :joins => :washing_instructions, :conditions => ['washing_instructions.dry_clean_only = ?', true]
  end

The above calls to named_scope define class methods Shirt.red and Shirt.dry_clean_only. Shirt.red, in effect, represents the query Shirt.find(:all, :conditions => {:color => 'red'}).

Unlike Shirt.find(...), however, the object returned by Shirt.red is not an Array; it resembles the association object constructed by a has_many declaration. For instance, you can invoke Shirt.red.find(:first), Shirt.red.count, Shirt.red.find(:all, :conditions => {:size => 'small'}). Also, just as with the association objects, named \scopes act like an Array, implementing Enumerable; Shirt.red.each(&block), Shirt.red.first, and Shirt.red.inject(memo, &block) all behave as if Shirt.red really was an Array.

These named \scopes are composable. For instance, Shirt.red.dry_clean_only will produce all shirts that are both red and dry clean only. Nested finds and calculations also work with these compositions: Shirt.red.dry_clean_only.count returns the number of garments for which these criteria obtain. Similarly with Shirt.red.dry_clean_only.average(:thread_count).

All \scopes are available as class methods on the ActiveRecord::Base descendent upon which the \scopes were defined. But they are also available to has_many associations. If,

  class Person < ActiveRecord::Base
    has_many :shirts
  end

then elton.shirts.red.dry_clean_only will return all of Elton’s red, dry clean only shirts.

Named \scopes can also be procedural:

  class Shirt < ActiveRecord::Base
    named_scope :colored, lambda { |color|
      { :conditions => { :color => color } }
    }
  end

In this example, Shirt.colored('puce') finds all puce shirts.

Named \scopes can also have extensions, just as with has_many declarations:

  class Shirt < ActiveRecord::Base
    named_scope :red, :conditions => {:color => 'red'} do
      def dom_id
        'red_shirts'
      end
    end
  end

For testing complex named \scopes, you can examine the scoping options using the proxy_options method on the proxy itself.

  class Shirt < ActiveRecord::Base
    named_scope :colored, lambda { |color|
      { :conditions => { :color => color } }
    }
  end

  expected_options = { :conditions => { :colored => 'red' } }
  assert_equal expected_options, Shirt.colored('red').proxy_options
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July 24, 2008 - (v2.1.0)
20 thanks

automatically generate scopes for model states

or better known as “throw on some more tasty meta-programming” :). Given an example of a model which has a state (String) which must from a set of defined values, e.g. pending, approved, denied.

class User < ActiveRecord::Base
  STATES = [ 'pending', 'approved', 'denied' ]

  validates_inclusion_of :state, :in => STATES

  # Define a named scope for each state in STATES
  STATES.each { |s| named_scope s, :conditions => { :state => s } }
end

This automatically defines a named_scope for each of the model states without having to define a named_scope manually for each state (nice and DRY).

June 27, 2008
15 thanks

Passing find() arguments

I you need to pass additional arguments to a scope (e.g. limit), do this:

Shirt.colored('red').all(:limit => 10)
March 10, 2009 - (>= v2.1.0)
8 thanks

Use lambda to avoid caching of generated query

If you’re using a named_scope that includes a changing variable you need to wrap it in a lambda to avoid the query being cached and thus becoming unaffected by future changes to the variable, example:

named_scope :translated, :conditions => { :locale => I18n.locale }

Will always return the same locale after the first hit even though I18n.locale might change. So do this instead:

named_scope :translated, lambda { { :conditions => { :locale => I18n.locale } } }

Ugly, but at least it’s working as we expect it…

December 12, 2008
7 thanks

acts_as_state_machine named scopes

If you are using the acts_as_state_machine plugin, this will generate all named scopes for your various states.

Place it after the acts_as_state_machine and state declarations.

class Task < ActiveRecord::Base
  acts_as_state_machine :initial => :waiting
  state :waiting
  state :running
  state :finished    

  states.each { |s| named_scope s, :conditions => { :state => s.to_s } }
end

Then doing a Task.waiting will return the corresponding tasks.

March 21, 2009
4 thanks

Passing optional arguments with defaults to a named_scope

An easy way to do this. (This also shows how you can use joins in a named_scope as well.)

Class User << ActiveRecord::Base
belongs_to :semester 

named_scope :year, lambda { |*year|
  if year.empty? || year.first.nil?
    { :joins => :semester, :conditions => ["year = #{CURRENT_SEMESTER}"]}
  else
    { :joins => :semester, :conditions => ["year = #{year}"]}
  end
  }

end

You can then call:

User.year     # defaults to CURRENT_SEMESTER constant
User.year()  # same as above
User.year(nil)  # same as above; useful if passing a param value that may or may not exist, ie, param[:year]
User.year(2010)
February 16, 2010 - (>= v2.1.0)
3 thanks

Extract the aggregated scoping options

If you want to get the aggregated scoping options of a chain of named scopes use ActiveRecord::Base.current_scoped_methods

It works in the fashion of:

Shirt.red.medium.alphabetical.current_scoped_methods
# ==>
{
  :create => {}, 
  :find => {
    :conditions => {:color => 'red', :size => 'medium'}, 
    :order => 'shirts.name ASC'
  } 
}
May 18, 2010
2 thanks

How using Array methods

It’s not possible to use Array methods with a scope because it’s not an Array but an ActiveRecord::NamedScope::Scope :

this

Article.promotion.sum(&:price)

doesn’t run.

But you can use the to_a method to transform the ActiveRecord::NamedScope::Scope to an Array :

Article.promotion.to_a.sum(&:price)
March 4, 2010
1 thank

AASM named scopes

If you are using the aasm plugin/gem, this will generate all named scopes for your various states.

Code example

Class Article <  ActiveRecord::Base

  include AASM

  aasm_initial_state :created

  aasm_state :published
  aasm_state :unpublished
  aasm_state :deleted
  aasm_state :created

  aasm_event :publish do
    transitions :to => :published, :from => [:created]
  end

  aasm_event :unpublish do
    transitions :to => :unpublished, :from => [:created, :published]
  end

  aasm_event :delete do
    transitions :to => :deleted, :from => [:published, :unpublished]
  end

  aasm_states.each { |s| named_scope s, :conditions => { :state => s.to_s } }  

end
March 23, 2010
1 thank

named_scopes and Acts as State Machine

As of AASM 2.1 named_scope(s) are defined by default for each state of a model.

March 21, 2009
1 thank

Generating empty conditions

In some cases, you might find it useful for your lamba to generate empty conditions based on the passed parameter.

Class Article << ActiveRecord::Base

named_scope :category, lambda { |cat|
  if cat == :all
    { :conditions => {} }
  else
    { :conditions => { :category_id => cat } }
  end  
}

end

Allows you to call something like this:

categories = user_is_admin ? :all : @current_category
Article.category(categories)

Mostly useful when chaining named_scopes together. Avoids more complicated if statements.

August 4, 2010 - (>= v2.0.0)
1 thank

Testing Named Scopes

Thanks for the example of testing named_scopes. Being new to Rails, I struggled to find examples that I could understand. Here is another “simple” test for a named_scope

Mine differs slightly from the one above in that I had to remove a set of {} in the :conditions in my test to avoid an “odd number list for Hash” error. I also replace the param-binding “?” with the number I expect to send in as an argument. My test would did know what args[0] was. I got an “undefined local variable” error.

The named scope in my model:

named_scope :up_to_and_including_year, lambda{ |*args| {

:conditions => [“to_char(grad_dt1,‘YYYY’) <= ?”, args[0]] }}

The test:

test "named_scope :up_to_and_including_year" do
  expected_options = { :conditions =>  ["to_char(grad_dt1,'YYYY') <= ?", '2010'] }
  assert_equal expected_options, Sso::CourseTaken.up_to_and_including_year('2010').proxy_options
end
September 21, 2010
1 thank

using joins, group, having

Code example

named_scope :red, :joins =>   [:color_lis => :color], :group => "color.id", :having => ["color.xx IS NULL"]
October 31, 2010
1 thank

A named_scope also responds to model class methods

for instance

class Student < ActiveRecord::Base

   named_scope :sophomore, :conditions => 'year=2'

   def self.eligible_to_vote
     select{|s| s.age >= 18}
   end

end

ss = Student.sophomore.eligible_to_vote
October 30, 2008
0 thanks

Anyone know the order the scopes assemble conditions?

It seems like last scope = first condition in sql. Can anyone confirm?

March 21, 2009
0 thanks

Remember, named_scope returns an array

named_scope always returns a named_scope object, that acts like an array, even if you’re using it to only find one record. So if you’re trying to perform an association on the results of a named_scope, use the first method to return the model object and not the named_scope object.

Ie:

user = User.my_name_scope
user.articles   # assuming User has_many Articles

will return an error. use this instead:

user = User.my_named_scope.first
user.articles

(Of course this is a poor example because what you should be doing is performing the named_scope on Article with user as the condition, instead of on User. But if you do need to use the results of a named_scope to perform an association call, you have to do it this way to avoid an error.)

February 9, 2011 - (<= v2.3.8)
0 thanks

some gotchas

Works

named_scope :public, :conditions => "public = true"

Works

PUBLIC_CONDITIONS = "public = true"
named_scope :public, :conditions => SomeModel::PUBLIC_CONDITIONS

Works

named_scope :public, lamba { {:conditions => SomeModel.public_conditions} }
def self.public_conditions
  "public = true"
end

Doesn’t work

named_scope :public, :conditions => SomeModel.public_conditions
def self.public_conditions
  "public = true"
end