ActiveRecord Cheat Sheet v1

ActiveRecord cheat sheet / EXAMPLES

INSTALL 
=======  
$ gem install activerecord
in GEMFILE: gem ‘activerecord’

REQUIRE 
=======  
require ‘active_record’

HOW TO USE

ESTABLISH A DATABASE CONNECTION 
=======
ActiveRecord::Base.establish_connection( :adapter => 'sqlite3',    :database => 'database_path')

CREATE TABLE IN DATABASE
=======
ActiveRecord::Base.connection.execute <<-SQL
  CREATE TABLE IF NOT EXISTS students (
    id INTEGER PRIMARY KEY AUTOINCREMENT,
    first_name VARCHAR NOT NULL,
    last_name VARCHAR NOT NULL,
  );
SQL

RELATIONSHIPS 
======= 
one-to-one:

class Customer < ActiveRecord::Base 
has_one :detail 
end 

class Detail < ActiveRecord::Base
belongs_to :customer 
end

## to include automatic building of detail when instantiate a customer, add this to customer class:

has_one :detail 
after_initialize do 
self.build_detail 
end

## to automatically destroy detail record when destroy customer record:

has_one :detail, :dependent => :destroy

## to hide child object, add the following to parent class:

delegate :birthday, :gender, :city, :to => :detail
OR
delegate *Detail::ATTR_METHODS, :to => :detail

## (must include ATTR_METHODS = [:birthday, :birthday=, :gender, :gender=, :city, :city=] in detail class to use 2nd option]

one-to-many

class Parent < ActiveRecord::Base 
has_many :children 
end 

class Child < ActiveRecord::Base 
belongs_to :parent 
end

## to destroy chid objects when parent in destroyed, add the following to the parent class:

	has_many :children, :dependent => :destroy


many-to-many

class Magazine < ActiveRecord::Base 
	has_many :subscriptions 
	has_many :customers, :through => :subscriptions 
end 

class Customer < ActiveRecord::Base 
	has_many :subscriptions 
	has_many :magazines, :through => :subscriptions 
end

## this makes it easy to find a customer’s magazines:

	@customer = customer.find(17)
	@customer.magazines

## it also makes it easy to find a customer’s subscriptions:

	@customer.subscriptions

CREATING RECORDS:
=====================

Single/Parent:

myStudent1 = Student.new(:first_name => 'Ann', :last_name => 'Smith', :birthday => '1986-08-11')

	myStudent1.save 


Child:

	(myStudent1.classes.new(:name => ‘Geometry’)).save

	##note alternative method for saving and pluralized ‘classes’##



QUERIES 
=====================

Find one by id:

Customer.find(17)

## finds customer with an id of 17

Find related records by id: 

Customer.find(17).detail.birthday 

## finds birthday field for the customer with id = 17. Birthday field is contained in Details table.

Find by label:

Can do:
- Person.where(:user_name => user_name).first
- Person.where(:last_name => last_name).all
- Person.where(:user_name => user_name, :password => password).first

Better:
- Person.find_by_user_name(user_name)
- Person.find_all_by_last_name(last_name)
- Person.find_by_user_name_and_password(user_name, password)

List all:

Contact.all.each do |contact|
puts contact.first_name
end
   	
## puts of first name for each contact in Contacts table

Conditional:

Student.where(:gender => 'male')

## finds students with the value ‘male’ in the gender column

LIKE:

Contact.where("first_name LIKE 'A%'")
OR
Contact.where(‘first_name LIKE ?', “A%")

## both examples above find contacts where first_name column begins with A. Some people prefer second for stylistic reasons

## Using LIKE to find values in the middle of a string:

Student.where("birthday LIKE '%-07-%'").each do |student|
  puts student.first_name + " has a birthday in July"
end

## prints the first name of all students with birthdays in July

Count:

	Client.count
	## prints count of number of records in Client table

	Client.where(:first_name => 'Ryan').count

## prints the the count of clients where first_name is Ryan


Unique:

	Client.select(:name).uniq

## only grabs a single record per unique value in the :name field

## A more complicated example using .uniq:

contacts = Address.where(:state => 'CA').includes(:contact).collect do |address|
  address.contact
end

contacts.uniq.each do |contact|
  puts contact
end

## finds each address in CA and then prints out each contact with an address in CA only once

## Another way doing this (via http://whatcodecraves.com/articles/2009/04/08/adventures-with-active-record-find):

Fruit.all(:select => "DISTINCT fruits.*", ...)