Hands-on with Rails DBs and Models

Database schema for an event schedule

Person(name, dob, gender, zipcode)
Event(name, date)
Registrations(person_id, event_id, status, comment)

Create the database tables

rails generate resource Person name:string dob:date gender:string zipcode:string
rails generate resource Event name:string event_date:date event_time:time
rails generate model Registration person_id:integer event_id:integer

Define sample data in seeds.rb

Event.create(name: 'Welcome', event_date: 'Feb 1 2014', event_time: '12:00pm')
Person.create(name: 'Angelo Smith', dob: 'Jan 2, 1991', gender: 'f', zipcode: 02474)
Registration.create(person_id: 3, event_id: 2)

Commands relating to seeding

rake db:create  # creates the database for the current env
rake db:seed    # (only) runs the db/seed.rb file
rake db:setup   # runs db:schema:load, db:seed

Examining the DB from the SQL console

In the database console you can type classic SQL commands

rails dbconsole
.help
.databases
select * from people
select * from people where id > 1

Examining the databases from the Rails console

In the rails console you can type ruby commands (it’s pry)

ActiveRecord::Base.connection.tables
Person.all
Person.where(gender: 'm')
Person.where("dob > ?", Date.new(1995))
Person.where("dob > ?", Date.new(1995)).count

CRUD operations (Create, Read, Update, Delete)

p = Person.new(name: "Pito", dob: '12-12-2012', gender: 'f', zipcode: 12355)
p.new_record?
p.save
p = Person.create(name: "Amin", dob: '12-12-2014', gender: 'm', zipcode: 31231)
p.delete

Associations, declared in models

class Person < ActiveRecord::Base
  has_many :registrations
  has_many :events, through: :registrations
end
class Event < ActiveRecord::Base
  has_many :registrations
  has_many :people, through: :registrations
end
class Registration < ActiveRecord::Base
  belongs_to :person
  belongs_to :event
end

Association, used

Person.find(1).registrations
Person.where(name: "Angelo Smith").first.registrations
Event.find(1).people

Validations

Validations are applied by ActiveRecord, NOT the database

validates :zipcode, length: { is: 5}    # declared in model file
p = Person.find(1)
p.zipcode = 1                           # assign invalid value
p.valid?                                # explicit check
p.save                                  # Is not permitted into the database
p.save!                                 # Same but throws exception

Appendices

References

• Awesome Print
• ActiveRecord Associations
• ActiveRecord Queries
• ActiveRecord Queries Cribsheet
• ActiveRecord Validations
• Validations Cribsheet

Gemfile

gem 'pry-rails'
gem 'pry'
gem 'pry-debugger'
gem 'pry-stack_explorer'
gem 'awesome_print'
gem 'hirb'