The document outlines various aspects of using Object-Relational Mappers (ORMs) such as Active Record, DataMapper, and Sequel in Ruby applications. It highlights differences in their functionalities and performance, discusses patterns, and provides examples of database operations with each ORM. Additionally, it emphasizes the importance of modularity and ease of use when selecting an ORM for projects.

1. Databases - Have it
your way
Frederick Cheung - kgb
fred@texperts.com
http://www.spacevatican.org
Friday, 4 December 2009 1

2. kgb
• Operates a number of Directory Enquiry type
products in several countries
• Runs the 542542 ‘Ask Us Anything’ SMS service in
the US
• 542542 backend is handled by several Rails apps
Friday, 4 December 2009 2

3. Whats the difference between 1080i and 1080p?
kgb
• Operates a number of Directory Enquiry type
products in several countries
• Runs the 542542 ‘Ask Us Anything’ SMS service in
the US
• 542542 backend is handled by several Rails apps
Friday, 4 December 2009 2

4. Whats the difference between 1080i and 1080p?
kgb
What is the address for the sperm bank of Pittsburgh?
• Operates a number of Directory Enquiry type
products in several countries
• Runs the 542542 ‘Ask Us Anything’ SMS service in
the US
• 542542 backend is handled by several Rails apps
Friday, 4 December 2009 2

5. Whats the difference between 1080i and 1080p?
kgb
What is the address for the sperm bank of Pittsburgh?
• Operates a number of Directory Enquiry type
products in several countries
• Runs the Greyhound bus station in Rochester, NY?
Where is
the 542542 ‘Ask Us Anything’ SMS service in
the US
• 542542 backend is handled by several Rails apps
Friday, 4 December 2009 2

6. Whats the difference between 1080i and 1080p?
kgb
What is the address for the sperm bank of Pittsburgh?
• Operates a number of Directory Enquiry type
products in several countries
• Runs the Greyhound bus station in Rochester, NY?
Where is
the 542542 ‘Ask Us Anything’ SMS service in
the US
• 542542 backend is handled by several Rails apps
Is marijuana legal in Oregon?
Friday, 4 December 2009 2

7. Whats the difference between 1080i and 1080p?
kgb
What is the address for the sperm bank of Pittsburgh?
• Operates a number of Directory Enquiry type
products in several countries
• Runs the Greyhound bus station in Rochester, NY?
Where is
the 542542 ‘Ask Us Anything’ SMS service in
the US
• 542542 backend is handled by several Rails apps
Is marijuana legal in Oregon?
What is the longest life span of a dog in recorded history?
Friday, 4 December 2009 2

8. ORMs
• Hide the gunk that map objects to database
storage
• Eliminate a lot of the repetitive code
• don’t forget about not relational storage
(eg CouchDB)
Friday, 4 December 2009 3

9. Active Record within
Rails
• redirect_to @person
• form_for @person
• rake tasks for migrations
• automatically configured on app load
Friday, 4 December 2009 4

10. What ORM
agnosticism isn’t
• gsub(‘ActiveRecord’, ‘DataMapper’)
• Not trying to make all ORM libraries look
the same
Friday, 4 December 2009 5

11. What ORM
agnosticism isn’t
• gsub(‘ActiveRecord’, ‘DataMapper’)
• Not trying to make all ORM libraries look
the same
Friday, 4 December 2009 5

12. Then what is it?
• Document / codify interactions between
persistence layer and rest of Rails
• Grease the wheels
• Don’t make you feel like a second class
citizen
• Should be almost invisible to end user -
things should just work
Friday, 4 December 2009 6

13. Choose your ORM on its features
without worrying about bumps in
the road
Friday, 4 December 2009 7

14. Patterns
• 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)
• Data Mapper: “A layer of Mappers hat
moves data between objects and a database
while keeping them independent of each
other and the mapper itself.” (Martin
Fowler)
Friday, 4 December 2009 8

15. Some ORMs, side by
side
Active Record, DataMapper, Sequel
Friday, 4 December 2009 9

16. Similar features across
the board
• User.new(params[:user])
• Associations, scopes, migrations etc.
• Watch out for subtle differences!
Friday, 4 December 2009 10

17. Similar features across
the board
• User.new(params[:user])
• Associations, scopes, migrations etc.
• Watch out for subtle differences!
#Active Record
person.save! #=> raises if invalid
person.save #=> returns false if invalid
#Sequel
person.save #=> raises if invalid
person.save! #=> save ignoring validations
Friday, 4 December 2009 10

18. Active Record
• You all know it
• Does a lot of manipulation of SQL
fragments - very difficult to build an Active
Record adapter for non sql datasource
• SQL fragments make some things easy,
others awkward
Friday, 4 December 2009 11

19. Active Record Evolution
• In the beginning conditions were strings:
composing sets of conditions: yuck!
• hash conditions for equality & ranges
• named_scope
• and ...
Friday, 4 December 2009 12

20. Arel - Relation Algebra
• generates db queries
• Destined to underpin future version Active Record
• Operations all closed under composition
posts = Table(:posts)
posts.where(posts[:subject].eq('Bacon'))
posts.join(comments).on(posts[:id].eq(comments[:post_id]))
Friday, 4 December 2009 13

21. DataMapper
• Query objects: lazy loaded definition of a query
• Easy to compose queries
• Easy construction of complicated joins
• Modular design
all_posts = Post.all
about_test = all_posts.all :subject.like => '%test%'
with_comment_by_fred = about_test.all Post.comments.name => 'fred'
Friday, 4 December 2009 14

22. A DM using Model
• mappy feel to it
• explicit about attributes (versus checking schema)
class Post
include DataMapper::Resource
include DataMapper::Timestamps
property :id, Serial
property :subject, String, :nullable => false
property :body, Text, :nullable => false
property :created_at, DateTime, :nullable => false
property :updated_at, DateTime, :nullable => false
has n, :comments
end
Friday, 4 December 2009 15

23. Laziness
• attributes can be lazy loaded
• loads lazy loaded attributes in one query
for all results in collection
• property groups when defining laziness
• same strategy for loading associations
Post.get 1
=> #<Post @id=1 @subject="Hello world" @body=<not loaded> >
Friday, 4 December 2009 16

24. Laziness
• attributes can be lazy loaded
• loads lazy loaded attributes in one query
for all results in collection
• property groups when defining laziness
• same strategy for loading associations
Post.get 1
=> #<Post @id=1 @subject="Hello world" @body=<not loaded> >
Friday, 4 December 2009 16

25. Sequel
• Database toolkit - A ruby DSL for databases
DB[:posts].filter { created_at > Date::today - 1}.all
• ORM layer (Migrations, associations etc. - the usual)
• Master/Slave databases, sharding
• pure ruby DSL for conditions
• Mosts things are Datasets
Friday, 4 December 2009 17

26. Sequel Model
class Post < Sequel::Model
plugin :timestamps
one_to_many :comments
end
• comments_dataset
• loads of customisation for eager loads
• inverse associations
• flexible - join on non standard columns
Friday, 4 December 2009 18

27. Very modular
Friday, 4 December 2009 19

28. Very modular
Some high level
Friday, 4 December 2009 19

29. Very modular
Some high level
Identity map
Lazy attributes
Callbacks
Single/Class Table inheritance
Friday, 4 December 2009 19

30. Very modular
Some high level
Identity map
Lazy attributes
Callbacks
Single/Class Table inheritance
Some quite grungy
Friday, 4 December 2009 19

31. Very modular
Some high level
Identity map
Lazy attributes
Callbacks
Single/Class Table inheritance
Some quite grungy
Boolean readers
Association proxies
Friday, 4 December 2009 19

32. Monolithic vs Modular
• Active Record in theory modular, but no
easy way to choose
• DM explicitly modular: plugins implement
validations, migrations, ...
• Sequel::Model entirely plugin driven
• Modularity great when you know what
you’re doing, but ...
• Tyranny of choice?
Friday, 4 December 2009 20

33. Conditions
#Active Record
Post.all :conditions => ["subject like ?", "%bacon%"]
Post.all :conditions => {:approved => true}
Post.all :conditions => ["comments.name like ?", "%bacon%"],
:joins => :comments, :select => "distinct posts.*"
#DataMapper
Post.all :subject.like => '%bacon%'
Post.all Post.comments.name.like => '%fred%'
#Sequel
Post.filter {subject.like '%test%'}.all
Post.select('distinct posts.*').inner_join(:comments, :post_id
=> :id).filter {comments__name.like '%test%'}.all
Friday, 4 December 2009 21

34. More Finders
#Active Record
Post.find :all, :include => :comments, :order => 'created_at
desc'
#DataMapper
Post.all :order => [:created_at.desc]
#Sequel
Post.eager(:comments).order(:created_at.desc).all
Post.eager_graph(:comments).order('posts.created_at desc').all
Friday, 4 December 2009 22

35. You can use these
today
• You can already build apps with other data
stores
• You can use some of the niceties like
redirect_to @person
• You need to figure out dependencies
between Action Pack and Active Record
• Compatibility worries
Friday, 4 December 2009 23

36. Little niggles
• Action summary doesn’t count time in DM/
Sequel as database time
• Some manual setup
• have to define to_param
Friday, 4 December 2009 24

37. Code time!
Friday, 4 December 2009 25

38. That’s all folks!
fred@texperts.com
http://www.spacevatican.org
Friday, 4 December 2009 26