Databases - Have it your way - PDF Free Download

Databases - Have it your way Frederick Cheung - kgb fred@texperts.com http://www.spacevatican.org 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 2

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 2

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 Where is the Greyhound bus station in Rochester, NY? 542542 backend is handled by several Rails apps 2

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) 3

Active Record within Rails redirect_to @person form_for @person rake tasks for migrations automatically configured on app load 4

What ORM agnosticism isn t gsub( ActiveRecord, DataMapper ) Not trying to make all ORM libraries look the same 5

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 6

Choose your ORM on its features without worrying about bumps in the road 7

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) 8

Some ORMs, side by side Active Record, DataMapper, Sequel 9

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

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 10

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 11

Active Record Evolution In the beginning conditions were strings: composing sets of conditions: yuck! hash conditions for equality & ranges named_scope and... 12

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])) 13

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' 14

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 15

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> > 16

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 17

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 18

Very modular 19

Very modular Some high level 19

Very modular Some high level Identity map Lazy attributes Callbacks Single/Class Table inheritance 19

Very modular Some high level Identity map Lazy attributes Callbacks Single/Class Table inheritance Some quite grungy 19

Very modular Some high level Identity map Lazy attributes Callbacks Single/Class Table inheritance Some quite grungy Boolean readers Association proxies 19

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? 20

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 21

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 22

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 23

Little niggles Action summary doesn t count time in DM/ Sequel as database time Some manual setup have to define to_param 24

Code time! 25

That s all folks! fred@texperts.com http://www.spacevatican.org 26