active_record.rb

# Q8  write DB query with Ruby
#     mapping between model and table
#     mapping instance of model and a row in a table
#     mapping instance variable and columns of a table
#     ORM
# Q9  A ruby file which alter db schema structure (no data)
#     ex: table, column
# Q10
class CreateAuthors < ActiveRecord::Migration[5.1]
  def change
    create_table :authors do |t|
      t.string :name

      t.timestamps null: false
    end
  end
end

class CreateBooks < ActiveRecord::Migration[5.1]
  def change
    create_table :books do |t|
      t.string :title
      t.integer :publishing_year
      t.references :author, foreign_key: true

      t.timestamps null: false
    end
  end
end

class CreateUsers < ActiveRecord::Migration[5.1]
  def change
    create_table :users do |t|
      t.string :email

      t.timestamps null: false
    end
  end
end

class CreateReadings < ActiveRecord::Migration[5.1]
  def change
    create_table :readings do |t|
      t.date :date
      t.references :book, foreign_key: true
      t.references :user, foreign_key: true

      t.timestamps null: false
    end
  end
end

# Q11
class AddCategoryToBooks < ActiveRecord::Migration[5.1]
  def change
    add_column :books, :category, :string
  end
end

# Q12
class Author < ActiveRecord::Base
  has_many :books

  validates :name, presence: true
end

class Book < ActiveRecord::Base
  belongs_to :author
  has_many :readings
end

class User < ActiveRecord::Base
  has_many :readings
  has_many :books, through: :readings
end

class Reading < ActiveRecord::Base
  belongs_to :book
  belongs_to :user
end

# Q13
#1. Add your favorite author to the DB
Author.create(name: "Sir Arthur Conan Doyle")
#2. Get all authors
Author.all
#3. Get author with id=8
Author.find(8)
#4. Get author with name="Jules Verne", store it in a variable: jules
jules = Author.find_by(name: 'Jules Verne')
#5. Get Jules Verne's books
jules.books
#6. Create a new book "20000 Leagues under the Seas", it's missing # in the DB. Store it in a variable: twenty_thousand
twenty_thousand = Book.new(title: "20000 Leagues under the Seas")
#7. Add Jules Verne as this book's author
twenty_thousand.author = jules
#8. Now save this book in the DB!
twenty_thousand.save
# The end