Active Record Demystified

A brief overview of Active Record including some implementation details. Designed to explain some of the "magic" to beginners.

Transcript

1. Active Record Demystified Anthony Lewis
2. Any sufficiently advanced technology is indistinguishable from magic. Arthur C. Clark
3. Some people still say that Ruby on Rails has too much magic.
4. It’s not magic.
5. It’s Ruby
7. Active Record 1. Attributes 2. CRUD 3. Associations
8. Active Record An object that wraps a row in a database table or view, encapsulates the database access, and adds domain logic on that data. Martin Fowler, Patterns of Enterprise Application Architecture
9. Active Record in Rails • An implementation of the object-relational mapping (ORM) pattern of the same name • Automated mapping between classes and tables, attributes and columns • Associations between objects defined by simple class methods
10. Posts id title body 1 Hello, World Welcome to my blog... 2 My Cat The cutest kitty in the... 3 Too Busy Sorry I haven’t posted...
11. Posts id title body 1 Hello, World Welcome to my blog... 2 My Cat The cutest kitty in the... 3 Too Busy Sorry I haven’t posted... a post
12. Posts id title body 1 Hello, World Welcome to my blog... 2 My Cat The cutest kitty in the... 3 Too Busy Sorry I haven’t posted... an attribute
13. In Code # Not much to see here... class Post < ActiveRecord::Base end
14. At The Console >> Post class Post < ActiveRecord::Base { :id => :integer, :title => :string, :body => :text }
15. But How?
16. SQL
17. The Database mysql> SHOW FIELDS FROM `posts`; +--------+--------------+------+-- | Field | Type | Null | +--------+--------------+------+-- | id | int(11) | NO | | title | varchar(255) | YES | | body | text | YES | +--------+--------------+------+--
18. Attributes • Every ActiveRecord object stores the values for its attributes in a private variable called @attributes • ActiveRecord also adds methods for accessing each attribute such as title, title=, title?, etc.
19. Model Attributes p = Post.new p.title = “Hello, World” puts p.title # => Hello, World
20. Attribute Methods def define_method_attribute(attr) module_eval <<-STR def #{attr} read_attribute(#{attr}) end STR end
21. Read Attribute def read_attribute(attr) @attributes.fetch(attr) end
22. CRUD Create, Read, Update, Delete
23. Create p = Post.new p.title = “My Cat” p.save # or simply Post.create(:title => “My Cat”)
24. Read p = Post.find(2) p = Post.first p = Post.where(:title => “My Cat”) p = Post.order(‘title’).all p = Post.limit(5).all
25. Update p = Post.find(2) p.title = “2nd” p.save # or simply p = Post.find(2) p.update_attributes(:title=>“2nd”)
26. Delete p = Post.find(2) p.destroy Post.destroy_all
27. Dynamic Finders # how does this work? p = Post.find_by_title(“My Cat”)
28. Method Missing def method_missing(meth, *args) if meth.to_s =~ /^find_by_(.+)$/ attr = $1.split(‘_and_’) find_by_attributes(attr, *args) else super end end
29. Find By Attributes def find_by_attributes(attr, *args) conditions = Hash[attr.map { |a| [a, args[attr.index(a)]] }] where(conditions).first end
30. Associations Relationships Between Tables
31. Comments id author body post_id 1 Tom This is the worst... 1 2 Fred You are wrong... 1 3 Bob Lowest prices on... 1
32. Comments id author body post_id 1 Tom This is the worst... 1 2 Fred You are wrong... 1 3 Bob Lowest prices on... 1 foreign key
33. Relationships • Each Post has many Comments • Each Comment belongs to a Post
34. In Code class Post < ActiveRecord::Base has_many :comments end class Comment < ActiveRecord::Base belongs_to :post end
35. Associations • An association is a connection between two models • Implemented with macro-style calls that add methods to a model • Streamline the code required to maintain the relationship between models
36. In Code class Post < ActiveRecord::Base has_many :comments end class Comment < ActiveRecord::Base belongs_to :post end
37. belongs_to Methods • post • post= • build_post • create_post
38. has_many Methods • comments • comments<< • comments.delete • comments= • comments.clear • comments.empty? • comments.size • comments.find • comments.where • comments.exists? • comments.build • comments.create
39. Without Associations p = Post.find(1) c = Comment.new c.post_id = p.id c.author = “Tony” c.body = “A comment” c.save
40. With Associations p = Post.find(1) p.comments.create( :author => “Tony”, :body => “A comment” )
41. Resources • RailsGuides • http://guides.rubyonrails.org/ • Specifically: • Active Record Query Interface • Active Record Associations
42. Questions?