Arel in Rails 3
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This document discusses how Arel, the SQL abstraction library underlying ActiveRecord, brings relational algebra and operators to ActiveRecord queries. It provides examples of how relations can be combined and manipulated to perform more complex queries. Key points covered include the SELECT, WHERE, JOIN, and other SQL operators Arel supports and how ActiveRecord::Relations allow building queries incrementally prior to execution.
![Grouping and sorting data
> pubs_beers = Pub.joins(:beers)
> pubs = pubs_beers.select('pubs.name, count(*) as
beers_count').group('pubs.name')
> pubs.all[0].beers_count
# 2
> pubs.order('beers_count').all](https://image.slidesharecdn.com/arel-110204203323-phpapp01/85/Arel-in-Rails-3-7-320.jpg)
![Eager Loading Associations
Associations are lazily loaded by default and as such...
> pubs = Pub.all
> pubs[0].beers
> pubs[1].beers
...produces these queries to be executed:
SELECT pubs.* FROM pubs
SELECT * FROM beers
INNER JOIN beers_pubs
ON beers.id = beers_pubs.beer_id
WHERE (beers_pubs.pub_id = 1 )
SELECT * FROM beers
INNER JOIN beers_pubs
ON beers.id = beers_pubs.beer_id
WHERE (beers_pubs.pub_id = 2 )](https://image.slidesharecdn.com/arel-110204203323-phpapp01/85/Arel-in-Rails-3-10-320.jpg)
![Eager Loading Associations
Eager load what you need using the new relations syntax:
> pubs = Pub.includes(:beers).all
> pubs[0].beers
> pubs[1].beers
Note that only one query is used to load the Beers info:
SELECT pubs.* FROM pubs
SELECT beers.*, t0.pub_id as the_parent_record_id
FROM beers
INNER JOIN beers_pubs t0
ON beers.id = t0.beer_id
WHERE (t0.pub_id IN (1,2,3))](https://image.slidesharecdn.com/arel-110204203323-phpapp01/85/Arel-in-Rails-3-11-320.jpg)
Arel in Rails 3
- 1. Arel and Rails 3 Relational algebra meets ActiveRecord
- 2. What!? • Relational Algebra deals with sets of finite relations, which are closed under certain operators. These operators operate on one or more relations to yield a relation. • Relation (table) - Data structure composed of a heading (columns) and an unordered set of tuples (rows) that share the same type. • Closure - A set is said to be closed under some operation if the result of that operation is a member of the set. • e.g.: Real numbers are closed under subtraction but natural numbers are not. Although 3 and 7 are natural numbers the result of 3 - 7 is not.
- 3. Operators Primitive Derived Selection (WHERE) Set Intersection Projection (SELECT) Division Cross Join Natural Join Set Union Set Difference Rename (SELECT A as B)
- 4. ActiveRecord::Relation Example Pub.select(:name).where(:city => 'Sydney') #yields an ActiveRecord::Relation instance. No SQL ran. but prior to Rails 3... Pub.find(:all, :select => "name", :conditions => { :city => 'Sydney' }) #yields an Array with the results
- 5. Ok, but now what? Build more interesting queries by combining relations... > syd_pubs = Pub.select('pubs.name').where(:city => 'Sydney') > squire = Pub.joins(:beers).where(:beers => {:name => 'James Squire'}) Now get me all the pubs in sydney that sell James Squire! > (syd_pubs & squire).all # yields an Array
- 6. Arel in Action > (syd_pubs & squire).all This causes ActiveRecord::Relation to ask Arel to build the SQL query below, corresponding to the combined relations: SELECT pubs.name FROM pubs INNER JOIN beers_pubs ON beers_pubs.pub_id = pubs.id INNER JOIN beers ON beers.id = beers_pubs.beer_id WHERE (pubs.city = 'Sydney') AND (beers.name = 'James Squire')
- 7. Grouping and sorting data > pubs_beers = Pub.joins(:beers) > pubs = pubs_beers.select('pubs.name, count(*) as beers_count').group('pubs.name') > pubs.all[0].beers_count # 2 > pubs.order('beers_count').all
- 8. Range Conditions > today = Time.now.at_beginning_of_day > tomorrow = Time.now.tomorrow.at_beginning_of_day > Pub.where(:created_at => today..tomorrow).all Arel takes care of ranges as well... SELECT pubs.* FROM pubs WHERE (pubs.created_at BETWEEN '2010-10-05 00:00:00.000000' AND '2010-10-06 00:00:00.000000')
- 9. Scopes: Reusable Relations class Beer < ActiveRecord::Base scope :stout, where(:flavor => "Stout") end > Beer.stout.all # gives back an Array with the results
- 10. Eager Loading Associations Associations are lazily loaded by default and as such... > pubs = Pub.all > pubs[0].beers > pubs[1].beers ...produces these queries to be executed: SELECT pubs.* FROM pubs SELECT * FROM beers INNER JOIN beers_pubs ON beers.id = beers_pubs.beer_id WHERE (beers_pubs.pub_id = 1 ) SELECT * FROM beers INNER JOIN beers_pubs ON beers.id = beers_pubs.beer_id WHERE (beers_pubs.pub_id = 2 )
- 11. Eager Loading Associations Eager load what you need using the new relations syntax: > pubs = Pub.includes(:beers).all > pubs[0].beers > pubs[1].beers Note that only one query is used to load the Beers info: SELECT pubs.* FROM pubs SELECT beers.*, t0.pub_id as the_parent_record_id FROM beers INNER JOIN beers_pubs t0 ON beers.id = t0.beer_id WHERE (t0.pub_id IN (1,2,3))
- 12. Thank you!
- 13. Q&A
- 14. References • Arel - http://github.com/rails/arel • Rails - http://github.com/rails/rails • Rails Guides - http://guides.rubyonrails.org/ • Relational Algebra - http://en.wikipedia.org/wiki/Relational_algebra