Async database with databases library in FastAPI - Mini Project: Build & Apply

Async database with databases library

📖 Scenario: You are building a simple FastAPI app that connects to a SQLite database asynchronously using the databases library. This app will store and retrieve user names.

🎯 Goal: Create a FastAPI app that connects asynchronously to a SQLite database using the databases library. You will define the database URL, create a database instance, write an async function to fetch all users, and finally add the startup and shutdown events to connect and disconnect the database.

📋 What You'll Learn

Use the databases library to handle async database connections

Use SQLite as the database with the URL sqlite:///./test.db

Create a database variable for the connection

Write an async function fetch_users that fetches all users from a users table

Add FastAPI startup and shutdown event handlers to connect and disconnect the database

💡 Why This Matters

🌍 Real World

Many web applications need to access databases asynchronously to handle many users efficiently without blocking the server.

💼 Career

Knowing how to use async database libraries like databases with FastAPI is valuable for backend developer roles focused on modern Python web frameworks.

1

Set up the database URL and import libraries

Import databases and FastAPI. Create a variable called DATABASE_URL and set it to "sqlite:///./test.db".

FastAPI

# Your code here

Hint

Use import databases and from fastapi import FastAPI. Then assign the exact string "sqlite:///./test.db" to DATABASE_URL.

2

Create the database instance

Create a variable called database and assign it to databases.Database(DATABASE_URL).

FastAPI

from fastapi import FastAPI
import databases

DATABASE_URL = "sqlite:///./test.db"

# Create the database instance
# Your code here

Hint

Use databases.Database with the DATABASE_URL variable to create the database instance.

3

Write async function to fetch all users

Define an async function called fetch_users that returns the result of await database.fetch_all(query). Use the query "SELECT * FROM users" inside the function.

FastAPI

from fastapi import FastAPI
import databases

DATABASE_URL = "sqlite:///./test.db"
database = databases.Database(DATABASE_URL)

# Define async function to fetch users
# Your code here

Hint

Define an async function named fetch_users. Inside, set query to the exact string "SELECT * FROM users" and return await database.fetch_all(query).

4

Add FastAPI app and connect/disconnect events

Create a FastAPI app instance called app. Add @app.on_event("startup") decorated async function startup that calls await database.connect(). Add @app.on_event("shutdown") decorated async function shutdown that calls await database.disconnect().

FastAPI

from fastapi import FastAPI
import databases

DATABASE_URL = "sqlite:///./test.db"
database = databases.Database(DATABASE_URL)

async def fetch_users():
    query = "SELECT * FROM users"
    return await database.fetch_all(query)

# Create FastAPI app and add startup/shutdown events
# Your code here

Hint

Create app = FastAPI(). Use @app.on_event("startup") to decorate an async startup function that calls await database.connect(). Similarly, use @app.on_event("shutdown") to decorate an async shutdown function that calls await database.disconnect().

Practice

1. What is the main benefit of using the databases library with FastAPI for database operations?

easy

A. It automatically creates database tables without any code.

B. It allows running database queries asynchronously, keeping the app responsive.

C. It replaces FastAPI's routing system with database queries.

D. It makes the database run faster by caching all queries.

Solution

1. Step 1: Understand asynchronous behavior in FastAPI

   FastAPI supports async functions to avoid blocking operations, improving responsiveness.

2. Step 2: Role of the databases library

   The databases library allows async database queries, so the app doesn't wait for the database to respond before continuing.

3. Final Answer:

   It allows running database queries asynchronously, keeping the app responsive. -> Option B

4. Quick Check:

   Async queries keep app responsive = A [OK]

Hint: Async means non-blocking queries for smooth app flow [OK]

Common Mistakes:

• Thinking it auto-creates tables
• Confusing async with caching
• Believing it changes routing

2. Which of the following is the correct way to connect to the database using the databases library in FastAPI?

easy

A. await database.connection()

B. database.connect()

C. database.await_connect()

D. await database.connect()

Solution

1. Step 1: Recall async connection method

   The databases library requires awaiting the connect method because it is asynchronous.

2. Step 2: Identify correct syntax

   The correct syntax is await database.connect(). Calling without await or wrong method names causes errors.

3. Final Answer:

   await database.connect() -> Option D

4. Quick Check:

   Async connect uses await = A [OK]

Hint: Always await async connect calls in databases library [OK]

Common Mistakes:

• Forgetting to use await
• Using wrong method names
• Calling connect synchronously

3. Given this FastAPI code snippet using the databases library, what will be printed when the endpoint is called?

import databases
from fastapi import FastAPI

database = databases.Database('sqlite:///test.db')
app = FastAPI()

@app.on_event('startup')
async def startup():
    await database.connect()

@app.on_event('shutdown')
async def shutdown():
    await database.disconnect()

@app.get('/')
async def read_data():
    query = 'SELECT 1 as number'
    result = await database.fetch_one(query)
    print(result['number'])
    return {'number': result['number']}

medium

A. Error: invalid query

B. None

C. 1

D. Error: database not connected

Solution

1. Step 1: Check database connection lifecycle

   The database connects on startup and disconnects on shutdown, so it is connected when the endpoint runs.

2. Step 2: Analyze query and fetch_one result

   The query selects the number 1 as 'number'. The fetch_one returns a dict-like object with key 'number' and value 1.

3. Final Answer:

   1 -> Option C

4. Quick Check:

   Query returns 1 = D [OK]

Hint: Check connection events and query result keys [OK]

Common Mistakes:

• Assuming no connection established
• Expecting None instead of 1
• Misreading query syntax

4. Identify the error in this FastAPI code using the databases library:

import databases
from fastapi import FastAPI

database = databases.Database('sqlite:///test.db')
app = FastAPI()

@app.on_event('startup')
async def startup():
    await database.connect()

@app.on_event('shutdown')
async def shutdown():
    await database.disconnect()

@app.get('/')
async def read_data():
    query = 'SELECT 1 as number'
    result = database.fetch_one(query)
    return {'number': result['number']}

medium

A. Missing await before database.fetch_one causing a runtime error

B. Database URL is incorrect

C. Missing database.connect() call

D. Using synchronous FastAPI endpoint instead of async

Solution

1. Step 1: Check usage of async database method

   The fetch_one method is async and must be awaited to get the result properly.

2. Step 2: Identify missing await

   Code calls database.fetch_one(query) without await, which causes a runtime error because the coroutine is not awaited.

3. Final Answer:

   Missing await before database.fetch_one causing a runtime error -> Option A

4. Quick Check:

   Async calls need await = C [OK]

Hint: Always await async database calls to avoid errors [OK]

Common Mistakes:

• Forgetting await on async calls
• Assuming fetch_one is synchronous
• Ignoring connection lifecycle

5. You want to fetch all users from a database asynchronously using the databases library in FastAPI. Which code snippet correctly fetches all rows and returns them as a list of dictionaries?

database = databases.Database('sqlite:///test.db')

async def get_users():
    query = 'SELECT * FROM users'
    # Which line correctly fetches all rows?
    ???
    return users

hard

A. users = await database.fetch_all(query)

B. users = database.fetch_all(query)

C. users = await database.fetch_one(query)

D. users = database.fetch_one(query)

Solution

1. Step 1: Understand fetch_all vs fetch_one

   fetch_all returns all rows as a list; fetch_one returns a single row.

2. Step 2: Use await with async fetch_all

   Since fetch_all is async, it must be awaited to get the result.

3. Final Answer:

   users = await database.fetch_all(query) -> Option A

4. Quick Check:

   Fetch all rows async with await = B [OK]

Hint: Use await with fetch_all to get all rows asynchronously [OK]

Common Mistakes:

• Using fetch_one to get all rows
• Forgetting await on async calls
• Calling fetch_all synchronously