Introduction to ActiveRecord

Pre-work

To get the most of out using Ruby for databases, we should do some configuration of our irb sessions. This will make printing of database objects more awesome

To achieve this we will install a gem and configure our ~/.irbrc file to configure our console with some nice features.

• Step 1: Install awesome_print

gem install awesome_print

• Step 2: Edit a file ~/.irbrc and update it's content

If not code, use your favorite editor

code ~/.irbrc

Add this content. Don't worry too much about what it does, or how it works.

#!/usr/bin/ruby
require 'irb/completion'
require 'irb/ext/save-history'
# Require awesome_print in our environment
begin
  require 'awesome_print'
rescue LoadError
end
# Keep an IRB history
IRB.conf[:SAVE_HISTORY] = 1000
IRB.conf[:HISTORY_FILE] = "#{ENV['HOME']}/.irb_history"
# Simple prompt mode
IRB.conf[:PROMPT_MODE] = :SIMPLE
# Configure awesome print if available
if defined?(AwesomePrint)
  require 'awesome_print/ext/active_support'
  require 'awesome_print/ext/active_record'
  AwesomePrint.irb!
  AwesomePrint.defaults = {
    plain: false,
    index: false,
    ruby19_syntax: true,
    color: {
      hash:       :pale,
      class:      :white,
      bigdecimal: :yellow,
      integer:    :yellow,
      fixnum:     :yellow,
      symbol:     :yellow,
    },
    indent: -2
  }
end
# If we are in Rails
if ENV['RAILS_ENV'] || defined?(Rails)
  # Customize the IRB prompt
  app_name = Rails.application.class.parent_name.downcase
  app_env  = Rails.env[0...3]
  IRB.conf[:PROMPT] ||= {}
  IRB.conf[:PROMPT][:RAILS] = {
    PROMPT_I: "#{app_name}> ",
    PROMPT_S: "#{app_name}* ",
    PROMPT_C: "#{app_name}? ",
    RETURN: "=> %s\n"
  }
  IRB.conf[:PROMPT_MODE] = :RAILS
end

Database setup

• Step 1: Make sure you have these tables created. You likely have these created if you have previously followed this lesson

CREATE TABLE movies (
  id               SERIAL PRIMARY KEY,
  title            TEXT NOT NULL,
  primary_director TEXT,
  year_released    INT,
  genre            TEXT
);
CREATE TABLE ratings (
  id SERIAL PRIMARY KEY,
  rating TEXT
);
ALTER TABLE movies ADD COLUMN rating_id INTEGER NULL REFERENCES ratings (id);
CREATE TABLE actors (
  id        SERIAL PRIMARY KEY,
  full_name TEXT NOT NULL,
  birthday  DATE
);
CREATE TABLE roles (
  id       SERIAL PRIMARY KEY,
  movie_id  INTEGER REFERENCES movies (id),
  actor_id  INTEGER REFERENCES actors (id)
);

• Step 2: See this lesson for INSERT INTO statements to populate the database

Using bundler

Bundler is the main package manager for Ruby. It is like npm.

In most Ruby applications we will be using Bundler for managing the dependencies of our code. Bundler is a very powerful tool that can add libraries to your code from the main repository at rubygems or from Github. Bundler also allows you to require some libraries in development mode, some in test, and others in production. It also has a very powerful and flexible version requirement langauge.

For now we are going to use Bundler in it's inline mode since we'll only be dealing with a single Ruby file. Soon we will use Bundler in it's typical mode by using a Gemfile

Use bundler to require the Postgres and ActiveRecord libraries

Create a ruby script (movies.rb) to contain our Ruby code

require 'bundler/inline'
gemfile do
  source 'https://rubygems.org'
  gem 'pg'
  gem 'activerecord'
end
require 'active_record'

Basic Active Record Setup

• NOTE: If you did not use the name suncoast_movies, replace "suncoast_movies" with the name of your database where the movies, actors, and roles tables were created.

# Tell Ruby we are going to use Bundler
require 'bundler/inline'
# Define which libraries to use
gemfile do
  # Use the official source (rubygems)
  source 'https://rubygems.org'
  # Use postgres library
  gem 'pg'
  # Use ActiveRecord ORM
  gem 'activerecord'
end
# Load up the Active Record code
require 'active_record'
# When ActiveRecord needs to log something, like what SQL
# it is generating, send it to "STANDARD OUTput" (our terminal)
ActiveRecord::Base.logger = Logger.new(STDOUT)
# Hey ActiveRecord, establish a connection to our database
ActiveRecord::Base.establish_connection(
  adapter: "postgresql",
  database: "suncoast_movies"
)

Setup models

class Movie < ActiveRecord::Base
end
class Rating < ActiveRecord::Base
end

Query

• Get all the movies
  • Movie.all
• Get the first movie
  • Movie.first
• Get the last movie
  • Movie.last
• Get a specific movie by id
  • Movie.find(42)
• Get a specific movie by title
  • Movie.find_by(title: "Star Wars")
• Get all movies released in 1984
  • Movie.where(year_released: 1984)
• Get all movies released after 1984
  • Movie.where("year_released > ?", 1984)
• Get all movies where the name contains aliens
  • Movie.where("title like ?", "%aliens%")

Insert

• Movie.create(title: "SDG: The Adventure", primary_director: "Suncoast", year_released: 2018, genre: "code")

Delete

• Movie.where("year_released > ?", 1984).delete_all
• movie = Movie.find(42) then movie.delete

Update

• movie = Movie.find(4)
• movie.title = "New Title"
• movie.save
• movie.update_attributes(title: "Newer Title", year_released: 2007)

Join

• Update the Movie model

class Movie < ActiveRecord::Base
  belongs_to :rating
end

Now we can write statements like these:

movie = Movie.first
p movie.rating

• If we now extend the Rating model like this:

class Rating < ActiveRecord::Base
  has_many :movies
end

we can write queries such as:

rating = Rating.first
p rating.movies

Many to Many Join

• We must define a model for the Role model that joins the Movie model and the Actor model

class Actor < ActiveRecord::Base
end
class Role < ActiveRecord::Base
  belongs_to :movie
  belongs_to :actor
end

• Now we can revise Movie and Actor to use the Role model as our join model, we call this has_many :through

class Actor < ActiveRecord::Base
  has_many :roles
  has_many :movies, through: :roles
end
class Movie < ActiveRecord::Base
  has_many :roles
  has_many :actors, through: :roles
end

• And we can write code such as:

movie = Movie.find_by(title: "The Lord of the Rings: The Two Towers")
p movie.actors
actor = Actor.find_by(full_name: "Martin Freeman")
p actor.movies

Sneak peek at using this for APIs

Let's get all the Movie objects and print the resulting array of these movies as JSON.

movies = Movie.all
movies_as_a_hash = movies.as_json
puts JSON.pretty_generate(movies_as_a_hash)

The movies.as_json turns the collection of ActiveRecord objects into a Ruby hash and then JSON.pretty_generate formats a nice, human readable, JSON string. Finally puts prints the results.

If we were doing this all at once this could look like:

puts JSON.pretty_generate(Movie.all.as_json)

Additional ActiveRecord resources

• Active Record Basics
  • Sections 1, 2, 3, 5
• Active Record Querying
  • Sections 1, 2, 3, 4, 5
• Active Record Associations
  • Sections 1, 2