Databases - Have it your way - Space Vatican · 2009/12/4 · • Database toolkit - A ruby DSL...

Databases - Have it your way

Frederick Cheung - kgb

[email protected]://www.spacevatican.org

1Friday, 4 December 2009

mailto:[email protected]


http://www.spacevatican.org


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


kgb




Whats the difference between 1080i and 1080p?


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What is the address for the sperm bank of Pittsburgh?


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Where is the Greyhound bus station in Rochester, NY?


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Is marijuana legal in Oregon?





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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 Recordperson.save! #=> raises if invalidperson.save #=> returns false if invalid

#Sequelperson.save #=> raises if invalidperson.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, :commentsend


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

• ORM layer (Migrations, associations etc. - the usual)

• Master/Slave databases, sharding

• pure ruby DSL for conditions

• Mosts things are Datasets

DB[:posts].filter { created_at > Date::today - 1}.all


Sequel Model

class Post < Sequel::Model plugin :timestamps one_to_many :commentsend

• comments_dataset• loads of customisation for eager loads• inverse associations• flexible - join on non standard columns


Very modular


Very modularSome high level


Very modular

Lazy attributes

Callbacks

Identity map

Single/Class Table inheritance

Some high level


Very modular

Lazy attributes

Callbacks

Identity map


Some high level

Some quite grungy


Very modular

Lazy attributes

Callbacks

Identity map


Some high level

Some quite grungy

Association proxies

Boolean readers


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 RecordPost.all :conditions => ["subject like ?", "%bacon%"]Post.all :conditions => {:approved => true}Post.all :conditions => ["comments.name like ?", "%bacon%"],

:joins => :comments, :select => "distinct posts.*"

#DataMapperPost.all :subject.like => '%bacon%' Post.all Post.comments.name.like => '%fred%'

#SequelPost.filter {subject.like '%test%'}.allPost.select('distinct posts.*').inner_join(:comments, :post_id => :id).filter {comments__name.like '%test%'}.all


More Finders

#Active RecordPost.find :all, :include => :comments, :order => 'created_at desc'

#DataMapper

Post.all :order => [:created_at.desc]

#Sequel

Post.eager(:comments).order(:created_at.desc).allPost.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!

[email protected]://www.spacevatican.org






Databases - Have it your way - Space Vatican · 2009/12/4 · • Database toolkit - A ruby DSL...

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