You're building a library management system for a city library with 50,000 books. Here's the catch — the library may own 5 copies of the same book. "Clean Code" by Robert Martin is one book, but there are 5 physical copies. How do you model this?

Code

// Book = metadata about a title
class Book {
  isbn: string
  title: string
  author: string
  subject: string
  publishDate: Date
}

// BookItem = one physical copy of a Book
class BookItem {
  barcode: string
  book: Book
  status: BookStatus
  rack: RackLocation
  dueDate: Date | null
  borrower: Member | null
}

enum BookStatus {
  AVAILABLE, CHECKED_OUT, RESERVED, LOST
}

Question

How should you separate the concept of a book from its physical copies?

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Design Library System

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Book & BookItem Classes

Composition & Normalization

Code

// Book = metadata about a title
class Book {
  isbn: string
  title: string
  author: string
  subject: string
  publishDate: Date
}

// BookItem = one physical copy of a Book
class BookItem {
  barcode: string
  book: Book
  status: BookStatus
  rack: RackLocation
  dueDate: Date | null
  borrower: Member | null
}

enum BookStatus {
  AVAILABLE, CHECKED_OUT, RESERVED, LOST
}

Question

How should you separate the concept of a book from its physical copies?