Databases -- Have it Your Way (Frederick Cheung)

Databases - Have it
your way
Frederick Cheung - kgb

fred@texperts.com
http://www.spacevatican.org

Friday, 4 December 2009 1

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


Whats the difference between 1080i and 1080p?
kgb

the US


kgb
What is the address for the sperm bank of Pittsburgh?

the US


kgb

• Runs the Greyhound bus station in Rochester, NY?
Where is
the 542542 ‘Ask Us Anything’ SMS service in
the US


kgb

Where is
the US
Is marijuana legal in Oregon?


kgb

Where is
the US
Is marijuana legal in Oregon?

What is the longest life span of a dog in recorded history?

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)


Active Record within
Rails
• redirect_to @person
• form_for @person
• rake tasks for migrations
• automatically conﬁgured on app load


What ORM
agnosticism isn’t

• gsub(‘ActiveRecord’, ‘DataMapper’)
• Not trying to make all ORM libraries look
the same


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


Choose your ORM on its features
without worrying about bumps in
the road


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)


Some ORMs, side by
side

Active Record, DataMapper, Sequel


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


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


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


Active Record Evolution

• In the beginning conditions were strings:
composing sets of conditions: yuck!
• hash conditions for equality & ranges
• named_scope
• and ...


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]))


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'


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


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> >


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

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

Very modular


Very modular
Some high level


Very modular
Some high level
Identity map
Lazy attributes
Callbacks
Single/Class Table inheritance


Very modular
Some high level
Identity map
Lazy attributes
Callbacks
Some quite grungy


Very modular
Some high level
Identity map
Lazy attributes
Callbacks
Some quite grungy
Boolean readers
Association proxies

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?

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


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


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

Little niggles

• Action summary doesn’t count time in DM/
Sequel as database time
• Some manual setup
• have to deﬁne to_param


Code time!


That’s all folks!

fred@texperts.com
http://www.spacevatican.org


Databases -- Have it Your Way (Frederick Cheung)

Recommended

More Related Content

Similar to Databases -- Have it Your Way (Frederick Cheung) (20)

More from Skills Matter (20)

Recently uploaded (20)

Databases -- Have it Your Way (Frederick Cheung)