Loans

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Overview

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Checkout Books

async def checkout_books(books: list, user_id: int) -> None:
    """Mark each book as 'Checked Out' and create a loan record.
    
    Due in 14 days.
    """

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Parameters

books = [
    {"isbn": "9780132350884", "title": "Clean Code", ...},
    {"isbn": "9780201633610", "title": "Design Patterns", ...}
]

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Returns

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Side Effects

1. Updates Book Status: Sets each book’s status to “Checked Out”
2. Creates Loan Records: Inserts one loan record per book with:
   • user_id: The borrowing user
   • isbn: The book identifier
   • checked_out: Current timestamp (auto-set)
   • due_date: Current date + 14 days
   • returned: 0 (active loan)

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Implementation Details

due = datetime.now() + timedelta(days=14)

for book in books:
    # Update book status
    conn.execute(
        "UPDATE books SET status = 'Checked Out' WHERE isbn = ?",
        (book["isbn"],)
    )
    # Create loan record
    conn.execute(
        "INSERT INTO loans (user_id, isbn, due_date) VALUES (?, ?, ?)",
        (user_id, book["isbn"], due)
    )

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Return Book

async def return_book(isbn: str) -> bool:
    """Mark a book as Available and close its open loan."""

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Parameters

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Returns

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Operation Details

1. Find Active Loan: Queries for the most recent loan where:
   • isbn matches the parameter
   • returned = 0 (loan is active)
   • Orders by checked_out DESC to get the latest loan
2. Close Loan: If found, updates the loan record:
   • Sets returned = 1
   • Sets returned_date to current timestamp
3. Update Book: Sets book status back to “Available”

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Error Cases

• No active loan exists for the ISBN
• Book was never checked out
• Book was already returned

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Renew Book

async def renew_book(loan_id: int) -> bool:
    """Extend the due date of an active loan by 14 days from today."""

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Parameters

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Returns

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Renewal Logic

1. Verify Active Loan: Checks that loan exists and returned = 0
2. Calculate New Due Date: new_due_date = today + 14 days
3. Update Record: Sets due_date to the new date

• Renewing early gives 14 days from today
• Overdue books would get 14 days from today (if allowed)

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Business Logic Recommendations

• Disable renewal for overdue books
• Limit number of renewals per loan
• Only allow renewals within certain timeframes

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Get User Loans

async def get_user_loans(user_id: int) -> list:
    """Return all loans for a user, joined with book info."""

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Parameters

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Returns

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SQL Query

SELECT
    l.id, l.returned, l.due_date, l.returned_date,
    b.title, b.author, b.cover
FROM loans l
JOIN books b ON l.isbn = b.isbn
WHERE l.user_id = ?
ORDER BY l.checked_out DESC

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Date Parsing

• SQLite stores datetimes as ISO 8601 strings
• Function attempts to parse due_date and returned_date as datetime objects
• Falls back to original string value if parsing fails

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Use Cases

• My Books Page: Display current checkouts and history
• Overdue Tracking: Filter by due_date < now()
• Renewal Interface: Show active loans with renewal options
• Reading History: Filter by returned = True