quizz.rb

# Q1 - What's a relational database?
a set of tables linked to each other thanks to a system of primary / foreign keys
# Q2 - What are the different "table relationships" you know?
1:n, 1 to many
n:n, many to many
1:1, one to one

# Q3 - Consider this e-library service. An author, defined by
# his name have several books.
# A book, defined by its title and publishing year,
# has one author only. What's this simple database scheme.
# Please draw it!


# Q4 - A user, defined by his email, can read several books.
# A book (e-book!!) can be read by several user.
# We also want to keep track of reading dates.
# Improve your e-library DB scheme with relevant tables
# and relationships.


# Q5 - What's the language to make queries to a database?
Structured Query Language => SQL

# Q6 - What's the simple query to get books written before 1985?
SELECT * FROM books
WHERE publishing_year < 1985

# Q7 - What's the simple query to get the 3 most recent books
# written by Jules Verne?
SELECT * FROM books b
JOIN authors a ON a.id = b.author_id
WHERE a.name = "Jules Verne"
ORDER BY b.publishing_year DESC
LIMIT 3;

# Q8 - What's the purpose of ActiveRecord?
Active Record is a design pattern that maps your objects to a relational database
This pattern is knows as ORM (Object Relational Mapping)
and its also a gem

# Q9 - What's a migration? How do you run a migration?
A script that modifies the database schema
rake db:migrate

# Q10 - Complete migrations to create your e-library database

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, index: 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, index: true
      t.references :user, index: true
      t.timestamps null: false
    end
  end
end


# Q11 - Write a migration to add a category column to the books
# table.

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

# Q12 - Define an ActiveRecord model for each table of your DB.
# Add the ActiveRecord associations between models.

class Author < ActiveRecord::Base
  has_many :books
end

class Book < ActiveRecord::Base
  has_many :readings
  belongs_to :author
  has_many :users, through: :readings
end

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

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

# Q13 - Complete the following code using the relevant
# ActiveRecord methods.
# 1. Add your favorite author to the DB
Author.create(name: "someone")

# 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 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

# Q14 - Add validations of your choice to the Author class.
# Can we save an object in DB if its validations do not pass? NO
class Author < ActiveRecord::Base
  has_many :books
  validates :name, presence: true
end