📚 Get All Books by a Specific Author

🎯 Retrieve all books written by a specific author using:

• A Path Parameter

• A Query Parameter

🎯 Expected Behavior

Here’s what our endpoint should do:

• 👉 If we type Author Two → return 2 books

• 👉 If we type Author One → return 1 book

• 👉 If we type a non-existent author → return 0 books

We will first implement the Path Parameter solution.

🛣️ Approach 1 — Using a Path Parameter

We will create a new endpoint that accepts the author's name directly in the URL path.

🗂️ books.py

# endpoint that accepts the author's name directly in the URL path.
@app.get("/books/byauthor/{author}")
async def read_books_by_author_path(author: str):
    books_to_return = []
    for book in BOOKS:
        if book.get('author').casefold() == author.casefold():
            books_to_return.append(book)
    return books_to_return

🔍 Step-by-Step Explanation

👉 1. Define the Route

"/books/byauthor/{author}"

• {author} is a dynamic path parameter

• Whatever value is passed in that position becomes the author variable

👉 2. Define the Function Parameter

async def read_books_by_author_path(author: str): 

• author is typed as a string

• FastAPI automatically extracts it from the URL

👉 3. Create a Filtered List

books_to_return = []

We create an empty list to store matching books.

👉 4. Loop Through BOOKS

for book in BOOKS:

We iterate through all books.

👉 5. Case-Insensitive Comparison

book.get('author').casefold() == author.casefold()

🧠 Why use casefold()?

• Ensures case-insensitive comparison

• "Author Two" and "author two" will match

👉 6. Return the Results

return books_to_return

We return all books that match the author.

▶️ Running the Application

Open your terminal and start the server:

uvicorn books2:app --reload

🌐 Testing in the Browser

Steps:

1. Go to /docs

2. Locate: GET /books/byauthor/{author}

3. Click Try it out

4. Enter Author Two

5. Click Execute

✔ You should see 2 books

Try:

• Author One → 1 book

• Unknown Author → 0 books (an empty list)

🔄 Approach 2 — Switching to a Query Parameter

Now, let’s modify the endpoint to use a query parameter instead.

👉 To do this, remove {author} from the path.

When there is no dynamic path parameter, FastAPI automatically treats function parameters as query parameters.

After modifying:

@app.get("/books/byauthor/")
async def read_books_by_author_path(author: str):

Now author becomes a query parameter.

❌ Unexpected Error — “Unprocessable Entity”

If you refresh Swagger and test:

/books/byauthor/?author=Author Two

You may see:

422 Unprocessable Entity

🤔 Why is this happening?

This brings us to a very important concept.

⚠️ Critical Concept — ORDER MATTERS in FastAPI

This endpoint:

@app.get("/books/byauthor/")

Conflicts with this one:

@app.get("/books/{book_author}/")
async def read_author_and_category(book_author: str, category: str):

🧠 Why the Conflict Happens

FastAPI reads routes from top to bottom.

When you define:

@app.get("/books/{book_author}/") <---

It matches ANY dynamic value after /books/.

So when you type:

/books/byauthor

FastAPI thinks:

book_author = "byauthor"

And expects another parameter (category).

When it doesn't receive it:

❌ It throws a validation error.

🔄 Fix — Fixing the Route Order

To solve this:

👉 Move the more specific route to the top.

❌ Before (Incorrect Order)

🗂️ books.py

@app.get("/books/{book_author}/")
...
@app.get("/books/byauthor/{author}")

✅ After (Correct Order)

🗂️ books.py

@app.get("/books/byauthor/")
...
@app.get("/books/{book_author}/")

🧠 Rule of Thumb:

Always place more specific endpoints above more generic ones

🌐 Testing Again in the Browser

Now:

• GET /books/byauthor/{author} works

• GET /books/{book_author}/ still works

• Filtering by category also works

Everything functions correctly.

🧠 Why This Matters

FastAPI processes routes chronologically.

If a general route appears first, it can:

• Intercept requests

• Consume dynamic values

• Cause validation failures

This is especially important when working in a single file.

📘 Path vs Query Parameter

🚨 FastAPI Rule to Remember

from fastapi import FastAPI, Body

app = FastAPI()

BOOKS = [
    {"title": "Title One", "author": "Author One", "category": "science"},
    {"title": "Title Two", "author": "Author Two", "category": "science"},
    {"title": "Title Three", "author": "Author Three", "category": "history"},
    {"title": "Title Four", "author": "Author Four", "category": "math"},
    {"title": "Title Five", "author": "Author Five", "category": "math"},
    {"title": "Title Six", "author": "Author Two", "category": "math"}
]

@app.get("/books")
async def read_all_books():
    return BOOKS

@app.get('/books/{book_title}')
async def read_book(book_title: str):
    for book in BOOKS:
        if book.get('title').casefold() == book_title.casefold():
            return book

@app.get("/books/")
async def read_category_by_query(category: str):
    books_to_return = []
    for book in BOOKS:
        if book.get('category').casefold() == category.casefold():
            books_to_return.append(book)
    return books_to_return

@app.get("/books/byauthor/")
async def read_books_by_author_path(author: str):
    books_to_return = []
    for book in BOOKS:
        if book.get('author').casefold() == author.casefold():
            books_to_return.append(book)
    return books_to_return

@app.get("/books/{book_author}/")
async def read_author_and_category(book_author: str, category: str):
    books_to_return = []
    for book in BOOKS:
        print('book', book)
        if book.get('author').casefold() == book_author.casefold() and \\
                book.get('category').casefold() == category.casefold():
            books_to_return.append(book)
    return books_to_return

@app.post("/books/create_book")
async def create_book(new_book=Body()):
    BOOKS.append(new_book)

@app.put("/books/update_book")
async def update_book(updated_book=Body()):
    for i in range(len(BOOKS)):
        if BOOKS[i].get('title').casefold() == \\
                updated_book.get('title').casefold():
            BOOKS[i] = updated_book

@app.delete("/books/delete_book/{book_title}")
async def delete_book(book_title: str):
    for i in range(len(BOOKS)):
        if BOOKS[i].get('title').casefold() == book_title.casefold():
            BOOKS.pop(i) 
            break