Second Commit

app/models/application_record.rb

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+class ApplicationRecord < ActiveRecord::Base
+  self.abstract_class = true
+end

app/models/book.rb

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+class Book < ApplicationRecord
+  has_many :users_books
+  has_many :users, through: :users_books
+
+  enum genre: [ :tech, :science, :nonfiction, :fiction, :philosophy ]
+
+  enum sub_genre: {
+    signal_processing: 0,
+    data_science: 1,
+    mathematics: 2,
+    economics: 3,
+    history: 4,
+    psychology: 5,
+    classic: 6,
+    computer_science: 7,
+    novel: 8,
+    science: 9,
+    autobiography: 10,
+    physics: 11,
+    objectivism: 12,
+    trivia: 13,
+    misc: 14,
+    poetry: 15,
+    education: 16,
+    philosophy: 17,
+    anthology: 18,
+    politics: 19,
+    comic: 20,
+    legal: 21
+  }, _prefix: :sub
+  
+  def description
+    str = ''
+    str += genre if genre
+    str += ' / ' if sub_genre
+    str += sub_genre if sub_genre
+    str += ', ' if author
+    str += "by #{author}" if author
+    str += ', ' if publisher
+    str += "published by #{publisher}" if publisher
+    str += ' - ' if available_copies > 0
+    str += "<b>#{available_copies} copies left</b>" if available_copies > 0
+    str.html_safe
+  end
+
+  def available_copies
+    copies - users_books.where(status: :borrowed).count
+  end
+end

app/models/library.rb

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+class Library < ApplicationRecord
+end

app/models/user.rb

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+class User < ApplicationRecord
+  # Include default devise modules. Others available are:
+  # :confirmable, :lockable, :timeoutable, :trackable and :omniauthable
+  devise :database_authenticatable, :registerable,
+         :recoverable, :rememberable, :validatable
+  
+  has_many :users_books
+  has_many :books, through: :users_books
+end

app/models/users_book.rb

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+class UsersBook < ApplicationRecord
+  belongs_to :user
+  belongs_to :book
+  validates_uniqueness_of :book_id, scope: :user_id
+
+  enum status: {
+    borrowed: 0,
+    returned: 1,
+    overdue: 2
+  }, _default:  :borrowed
+
+  before_save :assign_identifier
+
+  def assign_identifier
+    self.identifier = "#{book.title.first(5).upcase + SecureRandom.urlsafe_base64.upcase}"
+  end
+
+  def due_date
+    created_at + 7.days
+  end
+end