introduction to active record at mysql conference 2007

Introduction to Active Record

Evan ‘Rabble’ Henshaw-Plath evan@protest.net - Yahoo! Brickhouse

anarchogeek.com - testingrails.com

Active Record is a Design Pattern

An object that wraps a row in a database table or view, encapsulates the database access, and

adds domain logic on that data.

Active Recordthe Pattern

Active Record uses the most obvious approach,putting data access logic in the domain object.

- Martin Fowler

Person last_name first_name dependents_count

insert update

get_exemption is_flagged_for_audit? get_taxable_earnings?

One Class Per Table

CREATE TABLE `users` ( `id` int(11) NOT NULL auto_increment, `login` varchar(255), `email` varchar(255), `crypted_password` varchar(40), `salt` varchar(40), `created_at` datetime default NULL, `updated_at` datetime default NULL, PRIMARY KEY (`id`)) ENGINE=InnoDB;

class User < ActiveRecord::Base end

The DatabaseThe Model Code

One Object Per Row

There Are Other Ways To Do it

• Table Data Gateway

• Row Data Gateway

• Data Mapper

• The Anti-Patterns

Active Record is just one ‘Data Source Architectural Pattern’

Standard Active Record

• Direct mapping to the DB

• Class to table

• Object to row

• Simple, no relationship between objects

• Just a finder method with getters and setters

ActiveRecordthe ruby library

Active Record is a library builtfor Ruby on Rails.

Makes CRUD EasyCreateReadUpdateDelete

ActiveRecordthe ruby library

I have never seen an Active Record implementation as complete

or as useful as rails. - Martin Fowler

Rails’ ActiveRecord• DRY Conventions & Assumptions

• Validations

• Before and after filters

• Database Agnostic (mostly)

• Migrations

• Model relationships

• has_many, belongs_to, etc...

What ActiveRecord Likes• mapping class names to

table names

• pluralized table names

• integer primary keys

• classname_id foreign keys

• simple schemas

• single table inheritance

Active RecordDoesn’t Like • views

• stored procedures

• foreign key constraints

• cascading commits

• split or clustered db’s

• enums

The Basics

class User < ActiveRecord::Base end

./app/models/user.rb Loading a user

>> user_obj = User.find(2)=> #<User:0x352e8bc @attributes= {"salt"=>"d9ef...", "updated_at"=>"2007-04-19 10:49:15", "crypted_password"=>"9c1...", "id"=>"2", "remember_token"=>"a8d...", "login"=>"rabble", "created_at"=>"2007-04-19 10:49:15", "email"=>"evan@protest.net"}>

User Load (0.003175) SELECT * FROM users WHERE (users.id = 2) LIMIT 1

The SQL Log

The Find Method

Find is the primary method of Active Record

Examples: User.find(23) User.find(:first) User.find(:all, :offset => 10, :limit => 10) User.find(:all, :include => [:account, :friends]) User.find(:all, :conditions => [“category in (?), categories, :limit => 50) User.find(:first).articles

The Four Ways of Find

Find by id: This can either be a specific id (1), a list of ids (1, 5, 6), or an array of ids ([5, 6, 10]).

Find first: This will return the first record matched by the options used.

Find all: This will return all the records matched by the options used.

Indirectly: The find method is used for AR lookups via associations.

Understanding Find

Model#find(:all, { parameters hash }

What Find Does: * generates sql * executes sql * returns an enumerable (array like object) * creates an AR model object for each row

Find with :conditions

:conditions - An SQL fragment like "administrator = 1" or [ "user_name = ?", username ].

Student.find(:all, :conditions => [‘first_name = ? and status = ?’ ‘rabble’, 1])

New Style (Edge Rails Only)Student.find(:all, :conditions => {:first_name => “rabble”, :status => 1})

SQL Executed:SELECT * FROM students WHERE (first_name = 'rabble' and status = 1);

Order By

:order - An SQL fragment like "created_at DESC, name".

Student.find(:all, :order => ‘updated_at DESC’)

SQL Executed:SELECT * FROM users ORDER BY created_at;

Group By

:group - An attribute name by which the result should be grouped. Uses the GROUP BY SQL-clause.

Student.find(:all, :group => ‘graduating_class’)

SQL Executed:SELECT * FROM users GROUP BY graduating_class;

Limit & Offset:limit - An integer determining the limit on the number of rows that should be returned.

:offset- An integer determining the offset from where the rows should be fetched. So at 5, it would skip the first 4 rows.

Student.find(:all, :limit => 10, :offset => 0)

SQL Executed:SELECT * FROM users LIMIT 0, 10;

Joins

:joins - An SQL fragment for additional joins like "LEFT JOIN comments ON comments.post_id = id". (Rarely needed).

Student.find(:all, :join => "LEFT JOIN comments ON comments.post_id = id")

SQL Executed:SELECT users.* FROM users, comments LEFT JOIN comments ON comments.post_id = users.id;

Returns read only objects unless you say :readonly => false

Alternative Finds

find_by_sql

find_by_attribute_and_attribute2

find_or_create

Depreciated Find’sfind_firstfind_allfind_on_conditions

class Project < ActiveRecord::Base belongs_to :portfolio has_one :project_manager has_many :milestones has_and_belongs_to_many :categoriesend

The Four Primary Associations

belongs_tohas_onehas_manyhas_and_belongs_to_many

Associations

One to One has_one & belongs_to

Many to One has_many & belongs_to

Many to Many has_and_belongs_to_many has_many :through

Associations

One to One

Use has_one in the base, and belongs_to in the associated model.

class Employee < ActiveRecord::Base has_one :office end

class Office < ActiveRecord::Base belongs_to :employee # foreign key - employee_id end

One To One Example>> joe_employee = Employee.find_by_first_name('joe')SELECT * FROM employees WHERE (employees.`first_name` = 'joe') LIMIT 1=> #<Employee:0x36beb14 @attributes={"id"=>"1", "first_name"=>"joe", "last_name"=>"schmo", "created_at"=>"2007-04-21 09:08:59"}>

>> joes_office = joe_employee.officeSELECT * FROM offices WHERE (offices.employee_id = 1) LIMIT 1=> #<Office:0x36bc06c @attributes={"employee_id"=>"1", "id"=>"1", "created_at"=>"2007-04-21 09:11:44", "location"=>"A4302"}>

>> joes_office.employeeSELECT * FROM employees WHERE (employees.`id` = 1)=> #<Employee:0x36b6ef0 @attributes={"id"=>"1", "first_name"=>"joe", "last_name"=>"schmo", "created_at"=>"2007-04-21 09:08:59"}>

belongs_toOne to One Relationship. Use belong to when the foreign key is in THIS table.

• Post#author (similar to Author.find(author_id) )

• Post#author=(author) (similar to post.author_id = author.id)

• Post#author? (similar to post.author == some_author)

• Post#author.nil?

• Post#build_author (similar to post.author = Author.new)

• Post#create_author (similar to post.author = Author; post.author.save;

Defining belongs_toclass Employee < ActiveRecord::Base belongs_to :firm, :foreign_key => "client_of"

belongs_to :author, :class_name => "Person", :foreign_key => "author_id"

belongs_to :valid_coupon, :class_name => "Coupon",

:foreign_key => "coupon_id", :conditions => 'discounts > #{payments_count}'

belongs_to :attachable, :polymorphic => trueend

has_one

• Account#beneficiary (similar to Beneficiary.find(:first, :conditions => "account_id = #{id}"))

• Account#beneficiary=(beneficiary) (similar to beneficiary.account_id = account.id; beneficiary.save)

• Account#beneficiary.nil?

• Account#build_beneficiary (similar to Beneficiary.new("account_id" => id))

• Account#create_beneficiary (similar to b = Beneficiary.new("account_id" => id); b.save; b)

One to One Relationship. Use has_one when the foreign key is in the OTHER table.

Defining has_oneclass Employee < ActiveRecord::Base

# destroys the associated credit card has_one :credit_card, :dependent => :destroy # updates the associated records foreign key value to null rather than destroying it has_one :credit_card, :dependent => :nullify has_one :last_comment, :class_name => "Comment", :order => "posted_on" has_one :project_manager, :class_name => "Person", :conditions => "role = 'project_manager'"

has_one :attachment, :as => :attachable

end

One to ManyOne-to-manyUse has_many in the base, and belongs_to in the associated model.

class Manager < ActiveRecord::Base has_many :employeesend

class Employee < ActiveRecord::Base belongs_to :manager # foreign key - manager_idend

>> benevolent_dictator = Manager.find(:first, :conditions => ['name = "DHH"']) SELECT * FROM managers WHERE (name = "DHH") LIMIT 1=> #<Manager:0x369b7b8 @attributes={"name"=>"DHH", "id"=>"1", "created_at"=>"2007-04-21 09:59:24"}>

>> minions = benevolent_dictator.employees SELECT * FROM employees WHERE (employees.manager_id = 1)=> [#<Employee:0x36926a4 @attributes={"manager_id"=>"1", "id"=>"1", "first_name"=>"joe", "last_name"=>"schmo", "created_at"=>"2007-04-21 09:08:59"}>, #<Employee:0x36925f0 @attributes={"manager_id"=>"1", "id"=>"2", "first_name"=>"funky", "last_name"=>"monkey", "created_at"=>"2007-04-21 09:58:20"}>]

One to Many

has_many

• Firm#clients (similar to Clients.find :all, :conditions => "firm_id = #{id}")

• Firm#clients<<

• Firm#clients.delete

• Firm#client_ids

• Firm#client_ids=

• Firm#clients=

Augmenting the Model

has_many

• Firm#client.clear

• Firm#clients.empty? (similar to firm.clients.size == 0)

• Firm#clients.size (similar to Client.count "firm_id = #{id}")

• Firm#clients.find (similar to Client.find(id, :conditions => "firm_id = #{id}"))

• Firm#clients.build (similar to Client.new("firm_id" => id))

• Firm#clients.create (similar to c = Client.new("firm_id" => id); c.save; c)

has_many examples

class Employee < ActiveRecord::Base has_many :comments, :order => "posted_on" has_many :comments, :include => :author has_many :people, :class_name => "Person", :conditions => "deleted = 0", :order => "name" has_many :tracks, :order => "position", :dependent => :destroy has_many :comments, :dependent => :nullify has_many :tags, :as => :taggable has_many :subscribers, :through => :subscriptions, :source => :user has_many :subscribers, :class_name => "Person", :finder_sql => 'SELECT DISTINCT people.* ' + 'FROM people p, post_subscriptions ps ' + 'WHERE ps.post_id = #{id} AND ps.person_id = p.id ' + 'ORDER BY p.first_name'end

Many to Many

Simple Joiner Tablehas_and_belongs_to_many

Joiner Modelhas_many :through

has_and_belongs_to_many

The Simple Joiner Table Way

neglected by

rails-core

has_and_belongs_to_many

has_and_belongs_to_many

• Developer#projects

• Developer#projects<<

• Developer#projects.delete

• Developer#projects=

• Developer#projects_ids

• Developer#projects_ids=

• Developer#clear

Augmenting the Model

has_and_belongs_to_many

• Developer#projects.empty?

• Developer#projects.size

• Developer#projects.find(id) # Also find(:first / :all)

• Developer#projects.build #(similar to Project.new("project_id" => id))

• Developer#projects.create (similar to c = Project.new("project_id" => id); c.save; c)

habtm example create_table :developers do |t| t.column :name, :string t.column :created_at, :datetime end

create_table :projects do |t| t.column :name, :string t.column :created_at, :datetime end

create_table(:developers_projects, :id => false) do |t| t.column :developer_id, :integer t.column :project_id, :integer end

habtm example>> d = Developer.find(1) SELECT * FROM developers WHERE (developers.`id` = 1)=> #<Developer:0x32bc7dc @attributes={"name"=>"rabble", "id"=>"1", "created_at"=>nil}>

>> d.projects SELECT * FROM projects INNER JOIN developers_projects ON projects.id = developers_projects.project_id WHERE (developers_projects.developer_id = 1 )=> [#<Project:0x3257cc4 @attributes= {"name"=>"ragi", "project_id"=>"1", "id"=>"1", "developer_id"=>"1", "created_at"=>nil}>, #<Project:0x3257c10 @attributes= {"name"=>"acts_as_autenticated", "project_id"=>"3", "id"=>"3", "developer_id"=>"1", "created_at"=>nil}>]

has_many :through

DHH’s One True Way

of Many to Many

has_many :through

Full Joiner Model

has_many :through

class Appearance < ActiveRecord::Base belongs_to :dancer belongs_to :movieend

class Dancer < ActiveRecord::Base has_many :appearances, :dependent => true has_many :movies, :through => :appearancesend

class Movie < ActiveRecord::Base has_many :appearances, :dependent => true has_many :dancers, :through => :appearancesend

Validations

class User < ActiveRecord::Base validates_confirmation_of :login, :password validates_confirmation_of :email, :message => "should match confirmation" validates_format_of :email, :with => /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\Z/i, :on => :createend

Validations

• Keeping Data Clean

• In object validation of fields, calculated validations

• Instead of key constraints

• The database is for storage, the model is for the business logic

• Kinds of validations, custom validations, etc...

But Wait?

• Aren’t format, presence, relationship validations supposed to be the database’s job?

• Traditionally, yes.

• ActiveRecord does constraints in the model, not the database

But Why?

• Validations & Constraints are Business Logic

• Business logic should be in the model

• It makes things easy

• End users can get useful error messages

• Makes the postback pattern work well

Data Integrity?

• It’s still possible to do constraints in the db

• But it’s not as necessary

• Validations are constraints which make sense in terms of functionality of the app

• The rails ways is to just use validations

• Most DBA’s insist on foreign_key constraints

What AR Returns?

• Arrays of Model Objects

• Preselects and instantiates objects

• Nifty methods: to_yaml, to_xml, to_json

Output Formats#<Employee:0x36926a4 @attributes= {"manager_id"=>"1", "id"=>"1", "first_name"=>"joe", "last_name"=>"schmo", "created_at"=>"2007-04-21 09:08:59"}>

ruby - inspect--- !ruby/object:Employee attributes: manager_id: "1" id: "1" first_name: joe last_name: schmo created_at: 2007-04-21 09:08:59

to_yaml

{attributes: {manager_id: "1", id: "1", first_name: "joe", last_name: "schmo", created_at: "2007-04-21 09:08:59"}}

to_json<?xml version="1.0" encoding="UTF-8"?><employee> <created-at type="datetime">2007-04-21T09:08:59-07:00</created-at> <first-name>joe</first-name> <id type="integer">1</id> <last-name>schmo</last-name> <manager-id type="integer">1</manager-id></employee>

to_xml

Before & After Callbacks

* (-) save * (-) valid? * (1) before_validation * (2) before_validation_on_create * (-) validate * (-) validate_on_create * (3) after_validation * (4) after_validation_on_create * (5) before_save * (6) before_create * (-) create * (7) after_create * (8) after_save

class Subscription < ActiveRecord::Base before_create :record_signup

private def record_signup self.signed_up_on = Date.today end end

class Firm < ActiveRecord::Base # Destroys the associated clients and #people when the firm is destroyed before_destroy { |record| Person.destroy_all "firm_id = #{record.id}" } before_destroy { |record| Client.destroy_all "client_of = #{record.id}" } end

Optimizing AR

• Eager Loading

• Use Memecached

• Add index to your migrations

Security

Doing it Securely

class User < ActiveRecord::Base def self.authenticate_unsafely(user_name, password) find(:first, :conditions => "user_name = '#{user_name}' AND password = '#{password}'") end

def self.authenticate_safely(user_name, password) find(:first, :conditions => [ "user_name = ? AND password = ?", user_name, password ]) end

# Edge Rails Only (Rails 2.0) def self.authenticate_safely_simply(user_name, password) find(:first, :conditions => { :user_name => user_name, :password => password }) end end

Examples Stolen From: http://www.therailsway.com/2007/3/26/association-proxies-are-your-friend

Use The ActiveRecord Relationships

Anti-Pattern #1: Manually specifying the IDs when you construct the queries;def show unless @todo_list = TodoList.find_by_id_and_user_id(params[:id], current_user.id) redirect_to '/'end

Anti-Pattern #2: Querying globally, then checking ownership after the fact;def show @todo_list = TodoList.find(params[:id]) redirect_to '/' unless @todo_list.user_id = current_user.idend

Anti-Pattern #3: Abusing with_scope for a this simple case either directly, or in an around_filter.def show with_scope(:find=>{:user_id=>current_user.id}) do @todo_list = TodoList.find(params[:id]) endend

Best Practice: The most effective way to do this is to call find on the todo_lists association.def show @todo_list = current_user.todo_lists.find(params[:id])end

Create Via Association Proxies

The Bad Waydef create @todo_list = TodoList.new(params[:todo_list]) @todo_list.user_id = current_user.id @todo_list.save! redirect_to todo_list_url(@todo_list)end

A Better Way: Use association proxies for creation.def create @todo_list = current_user.todo_lists.create! params[:todo_list] redirect_to todo_list_url(@todo_list)end

The Best Practice - Handle exceptions for the user.def create @todo_list = current_user.todo_lists.build params[:todo_list] if @todo_list.save redirect_to todo_list_url(@todo_list) else render :action=>'new' endend Examples Stolen From: http://www.therailsway.com/2007/3/26/association-proxies-are-your-friend

Special Fields * created_at * created_on * updated_at * updated_on * lock_version * type * id

* #{table_name}_count * position * parent_id * lft * rgt * quote * template

Table Inheritance

Class Table Inheritance: Represents an inheritance hierarchy of classes with one table for each class1.

Single Table Inheritance: Represents an inheritance hierarchy of classes as a single table that has columns for all the fields of the various classes2.

Concrete Table Inheritance: Represents an inheritance hierarchy of classes with one table per concrete class in the hierarchy

STI - Single Table Inheritance Represents an inheritance hierarchy of classes as a single

table that has columns for all the fields of the various classes.

STI - Single Table Inheritance

class Company < ActiveRecord::Base; end class Firm < Company; end class Client < Company; end class PriorityClient < Client; end

CREATE TABLE `companies` ( `id` int(11) default NULL, `name` varchar(255) default NULL, `type` varchar(32) default NULL)

Company.find(:first) SELECT * FROM companies LIMIT 1;

Firm.find(:first) SELECT * FROM companies WHERE type = ‘firm’ LIMIT 1;

Legacy Databases

How to do legacy databases with Active Record?

http://sl33p3r.free.fr/tutorials/rails/legacy/legacy_databases.html

class CustomerNote < ActiveRecord::Base

set_primary_key "client_comment_id" set_sequence_name "FooBarSequences"

def self.table_name() "client_comment" end def body read_attribute "client_comment_body" end

def body=(value) write_attribute "client_comment_body", value endend

Supporting Legacy DB’s

Thanks to: http://www.robbyonrails.com/articles/2005/07/25/the-legacy-of-databases-with-rails

Changing ActiveRecord

Thanks to: http://fora.pragprog.com/rails-recipes/write-your-own/post/84

Modify Active Record ActiveRecord::Base.table_name_prefix = "my_" ActiveRecord::Base.table_name_suffix = "_table" ActiveRecord::Base.pluralize_table_names = false

Fixing the Auto-Increment / Sequence Problem module ActiveRecord class Base class << self def reset_sequence_name "#{table_name}_sequence" end end end end

module ActiveRecord module ConnectionAdapters class MysqlAdapter def prefetch_primary_key?(table_name = nil) true end end end end

Changing ActiveRecord

Thanks to: http://fora.pragprog.com/rails-recipes/write-your-own/post/84

Telling ActiveRecord to fetch the primary key

module ActiveRecord module ConnectionAdapters class MysqlAdapter

def prefetch_primary_key?(table_name = nil) true end

def next_sequence_value(sequence_name) sql = "UPDATE #{ sequence_name} SET Id=LAST_INSERT_ID(Id+1);" update(sql, "#{sequence_name} Update") select_value("SELECT Id from #{ sequence_name}",'Id') end

end end

Ruby on Rails AR Alternatives

Ruby DataMapper

iBatis - rBatis

Ruby DataMapperhttp://rubyforge.org/projects/datamapper

class FasterAuthor < DataMapper::Base

set_table_name 'authors'

property :name, :string, :size => 100 property :url, :string, :size => 255 property :is_active?, :boolean property :email, :string, :size => 255 property :hashed_pass, :string, :size => 40 property :created_at, :datetime property :modified_at, :datetime

has_many :posts, :class => 'FasterPost' # :foreign_key => 'post_id'

# prepends HTTP to a URL if necessary def self.prepend_http(url = '') if url and url != '' and not(url =~ /^http/i) url = 'http://' + url end return url end

end

iBatis - rBatisiBatis for Ruby (RBatis) is a port of Apache's iBatis library to Ruby and Ruby on Rails. It is an O/R-mapper that allows for complete customization of SQL. http://ibatis.apache.org

Not Very DRY / Rails Like

Drink the Kool aid?

Flickr Photos Used:http://flickr.com/photos/brraveheart/114402291/http://flickr.com/photos/bright/253175260/http://flickr.com/photos/good_day/63617697/http://flickr.com/photos/rickharris/416150393/http://flickr.com/photos/babasteve/3322247/http://flickr.com/photos/olivander/28058685/http://flickr.com/photos/brraveheart/44052308/http://flickr.com/photos/ednothing/142393509/http://flickr.com/photos/alltheaces/87505524/http://flickr.com/photos/alfr3do/7436142/http://flickr.com/photos/gdominici/57975123/http://flickr.com/photos/josefstuefer/72512671/http://flickr.com/photos/uqbar/105440294/http://flickr.com/photos/auntiep/17135231/http://flickr.com/photos/einsame_spitze/406992131/http://flickr.com/photos/beija-flor/63758047/http://flickr.com/photos/amerune/174617912/http://flickr.com/photos/hungry_i/47938311/http://flickr.com/photos/santos/13952912/http://flickr.com/photos/supermietzi/179962496/http://flickr.com/photos/traveller2020/206931940/http://flickr.com/photos/ko_an/318906221/

http://flickr.com/photos/ryangreenberg/57722319/http://flickr.com/photos/benandliz/11065337/http://flickr.com/photos/gaspi/12944421/http://flickr.com/photos/thomashawk/221827536/http://flickr.com/photos/brianboulos/7707518/http://flickr.com/photos/ross/28330560/http://flickr.com/photos/emdot/45249090/http://flickr.com/photos/farhang/428136695/http://flickr.com/photos/belljar/67877047/http://flickr.com/photos/pulpolux/34545782/http://flickr.com/photos/monkeyc/107979135/http://flickr.com/photos/pedrosimoes7/449314732/http://flickr.com/photos/dincordero/405452471/http://flickr.com/photos/andidfl/203883534/http://flickr.com/photos/ivanomak/434387836/http://flickr.com/photos/nrvica/23858419/http://flickr.com/photos/thespeak/137012632/http://flickr.com/photos/thowi/31533027/http://flickr.com/photos/thelifeofbryan/468557520/http://flickr.com/photos/eecue/289208982/http://flickr.com/photos/estherase/14110154/http://flickr.com/photos/ehnmark/118117670/

Introduction to Active RecordEvan ‘Rabble’ Henshaw-Plath

evan@protest.net - Yahoo! Brickhouseanarchogeek.com - testingrails.com

Questions?