Active Record Relation (AR::Relation) allows queries to be built up in steps and later executed all at once. It provides identity mapping, which caches objects in memory to ensure that multiple queries for the same object return the same object instance. Identity mapping was added by Emilio Tagua and provides performance and consistency benefits. Developers are encouraged to contribute to further improvements in AR::Relation and identity mapping.

1. Возможности,
особенности и
проблемы AR::Relation
Тимофей Цветков, Evil Martians
2011

2. Тимофей Цветков
2kan@evilmartians.com

3. Мотивации слайд

4. Я сказал убей!

5. Look at them!

6. SO HAPPY OMG

7. OMG LOL

8. ActiveRecord — не
черный ящик

9. Not only Arel
• Queries are not only Arel
• AR ≠ Arel

10. 3.x (x ≤1)
• It’s not only Arel! AR::Relation was released
• Identity Map (WHORRAAAA)

11. ActiveRecord
module ActiveRecord #:nodoc:
class << self # Class methods
delegate :find, :first, :first!, :last, :last!, :all, :exists?, :any?, :many?, :to
=> :scoped
delegate :destroy, :destroy_all, :delete, :delete_all, :update, :update_all, :to
=> :scoped
delegate :find_each, :find_in_batches, :to => :scoped
delegate :select, :group, :order, :except, :reorder, :limit, :offset, :joins, :where,
:preload, :eager_load, :includes, :from, :lock, :readonly, :having, :create_with, :to
=> :scoped
delegate :count, :average, :minimum, :maximum, :sum, :calculate, :to => :scoped
end
end

12. Scoped
def scoped(options = nil)
if options
scoped.apply_finder_options(options)
else
if current_scope
current_scope.clone
else
scope = relation.clone
scope.default_scoped = true
scope
end
end
end

13. Finder methods
• where
• having
• select
• order
• limit
• offset
All finder methods work through AR::Relation
• joins
• includes
• lock
• readonly
• from

14. Example.Where
def where(opts, *rest)
return self if opts.blank?
relation = clone
relation.where_values += build_where(opts, rest)
relation
end

15. ActiveRecord::Relation
def to_a
@records = if @readonly_value.nil? && !@klass.locking_enabled?
eager_loading? ? find_with_associations : @klass.find_by_sql
(arel.to_sql, @bind_values)
else
IdentityMap.without do
eager_loading? ? find_with_associations : @klass.find_by_sql
(arel.to_sql, @bind_values)
end
end
end
Real quering from the AR::Relation

16. AR::Relation problems
• merging default_scope’s with labmda (solved):
default_scope where(:name => "qwerty")
default_scope lambda { order("id DESC") }
• except (I’ve wrote a patch):
scope :xxx, order("id DESC")
scope :yyy, except(:order).order("id ASC")
@surveys = Survey.xxx.yyy
Survey Load (0.1ms) SELECT "surveys".* FROM "surveys" ORDER BY id DESC, id ASC

17. IdentityMap
Identity map was written by Emilio Tagua

18. Emilio Tagua
LEHAIM!

19. Without IdentityMap
user1 = User.find(1) # => #<User id: 1, name: "Josh">
user2 = User.find(1) # => #<User id: 1, name: "Josh">
user1 == user2 # => true, b/c AR::Base recognizes that
# they have the same primary key
user1.object_id == user2.object_id # => false, b/c these are two
# different in-memory objects
User Load (0.2ms) SELECT "users".* FROM "users" WHERE "users"."id" = ? LIMIT 1 [["id", 1]]
CACHE (0.0ms) SELECT "users".* FROM "users" WHERE "users"."id" = ? LIMIT 1 [["id", 1]]
Code example from: http://edgerails.info/articles/what-s-new-in-edge-rails/2011/04/21/activerecord-identity-
map/index.html

20. With IdentityMap
user1 = User.find(1) # => #<User id: 1, name: "Josh">
user2 = User.find(1) # => #<User id: 1, name: "Josh">
user1 == user2 # => true
user1.object_id == user2.object_id # => true, b/c these really are
# the same in-memory objects
User Load (2.2ms) SELECT "users".* FROM "users" LIMIT 1
User with ID = 1 loaded from Identity Map
Code example from: http://edgerails.info/articles/what-s-new-in-edge-rails/2011/04/21/activerecord-identity-
map/index.html

21. Even more...
post = Post.find(1)
same_post = post.comments.first.post
post.object_id == same_post.object_id # => true
Code example from: http://miloops.com/post/3391477665/identity-map-and-active-record

22. Thread safe
• Creates for each request
• Uses Thread.current
def repository
Thread.current[:identity_map] ||= Hash.new { |h,k| h[k] = {} }
end

23. Interesting...
def update
@survey = Survey.find(params[:id])
@surveys = Survey.all
if @survey.update_attributes(params[:survey])
redirect_to(@survey, :notice => 'Survey was successfully updated.')
else
render :action => "edit"
end
end
def update
@survey = Survey.find(params[:id])
if @survey.update_attributes(params[:survey])
redirect_to(@survey, :notice => 'Survey was successfully updated.')
else
@surveys = Survey.all
render :action => "edit"
end
end

24. How to use
IdentityMap
• It will be in Rails 3.1
• To enable write in your application.rb:
config.active_record.identity_map = true

25. Contribute!!!112233

26. Троллинг