associations.rb

# http://guides.rubyonrails.org/association_basics.html
require "pry"

require_relative "connection"

ActiveRecord::Schema.define do
  drop_table :users if ActiveRecord::Base.connection.data_source_exists? :users
  drop_table :groups if ActiveRecord::Base.connection.data_source_exists? :groups
  drop_table :hobbies if ActiveRecord::Base.connection.data_source_exists? :hobbies
  drop_table :group_memberships if ActiveRecord::Base.connection.data_source_exists? :group_memberships
  
  create_table :users do |t|
    t.string :name
    t.string :email
  end

  create_table :groups do |t|
    t.string :name
  end  

  create_table :group_memberships do |t|
    t.references :user
    t.references :group
    t.boolean :admin
  end
  
  create_table :hobbies do |t|
    t.string :name
    t.references :user
  end
end

class User < ActiveRecord::Base
  has_many :hobbies
  has_many :group_memberships
  has_many :groups, through: :group_memberships
end

class Hobby < ActiveRecord::Base
  belongs_to :user
end

class Group < ActiveRecord::Base
  has_many :group_memberships
  has_many :users, through: :group_memberships
end

class GroupMembership < ActiveRecord::Base
  belongs_to :user
  belongs_to :group
end

joel = User.create! name: 'Joel', email: '[email protected]'
mandolin = joel.hobbies.create! name: 'Mandolin'

p joel.hobbies.to_sql
joel.hobbies.each do |hobby|
  p hobby
end

instructors = Group.create! name: 'instructors'
p instructors.users.to_sql

instructors.group_memberships.create! user: joel, admin: true

puts "\nIterating Through Instructors Members"
instructors.users.each do |user|
  p user
end

callbacks.rb

# http://guides.rubyonrails.org/active_record_callbacks.html

require "pry"

require_relative "connection"

ActiveRecord::Schema.define do
  drop_table :users if ActiveRecord::Base.connection.data_source_exists? :users

  create_table :users do |t|
    t.string :name
    t.string :email
  end
end

class User < ActiveRecord::Base
  before_save :downcase_email # Before saving
  
  private
  def downcase_email
    self.email = email.downcase
  end
end

puts "\nCreating Joel with uppercase email"
joel = User.create! name: 'Joel', email: '[email protected]'
p joel.email

connection.rb

require "active_record"
# Output messages from Active Record to standard out
ActiveRecord::Base.logger = Logger.new(STDOUT)

puts 'Establishing connection to database ...'
ActiveRecord::Base.establish_connection(
    adapter: 'postgresql',
    database: 'ar_demo',
    username: 'development',
    password: 'development',
    host: 'localhost',
    port: 5432,
    pool: 5,
    encoding: 'unicode',
    min_messages: 'error'
)
puts 'CONNECTED'

Gemfile

# frozen_string_literal: true
source "https://rubygems.org"

gem "activerecord"
gem "pg"
gem "pry"

validations.rb

# http://guides.rubyonrails.org/active_record_validations.html

require "pry"

require_relative "connection"

ActiveRecord::Schema.define do
  drop_table :users if ActiveRecord::Base.connection.data_source_exists?(:users);
  drop_table :hobbies if ActiveRecord::Base.connection.data_source_exists?(:hobbies);
  create_table :users do |t|
    t.string :name
  end

  create_table :hobbies do |t|
    t.references :user
    t.string :name
  end  
end

class User < ActiveRecord::Base
  has_many :hobbies
  validate :nawar_is_so_mean # Custom validator method

  private
  def nawar_is_so_mean
    if self.name == "Nawar"
      errors.add :name, "Dude.  That was just.  I don't even know, man"
    end
  end
end

class Hobby < ActiveRecord::Base
  belongs_to :user
  validates :user, presence: true # Stock validation from ActiveRecord
  validate :two_of_the_same_hobby # Custom validator method

  private
  def two_of_the_same_hobby
    if self.user.hobbies.where(name: self.name).count > 0
      errors.add :name, "That hobby already exists: #{self.name}"
    end
  end
end

# This should not create
nawar = User.create name: 'Nawar'

joel = User.create! name: 'Joel'
joel.hobbies.create! name: 'Mandolin'
puts "\n Created 1 Mandolin"
# This should cause an error.
joel.hobbies.create! name: 'Mandolin'
puts "\n Created 2 Mandolins"