Async database with databases library in FastAPI

Introduction

Using async database calls lets your app handle many requests smoothly without waiting for the database. The databases library helps you do this easily with async support.

When building a web app that needs to handle many users at the same time.

When you want your app to stay fast even if database queries take time.

When using FastAPI or other async frameworks that support async code.

When you want simple code to connect and query databases asynchronously.

Syntax

FastAPI

import databases

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

async def connect_db():
    await database.connect()

async def disconnect_db():
    await database.disconnect()

async def fetch_data():
    query = "SELECT * FROM tablename"
    results = await database.fetch_all(query=query)
    return results

Use await to wait for database operations without blocking.

Call connect() before queries and disconnect() when done.

Examples

Example connecting to a PostgreSQL database asynchronously.

FastAPI

import databases

DATABASE_URL = "postgresql+asyncpg://user:pass@localhost/dbname"
database = databases.Database(DATABASE_URL)

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

Example inserting data asynchronously with parameters.

FastAPI

async def add_user(name: str):
    query = "INSERT INTO users(name) VALUES (:name)"
    await database.execute(query=query, values={"name": name})

Sample Program

This example creates a simple notes table, inserts one note, then fetches and prints all notes asynchronously using the databases library with SQLite.

FastAPI

import databases
import sqlalchemy
import asyncio

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

metadata = sqlalchemy.MetaData()

notes = sqlalchemy.Table(
    "notes",
    metadata,
    sqlalchemy.Column("id", sqlalchemy.Integer, primary_key=True),
    sqlalchemy.Column("text", sqlalchemy.String),
)

engine = sqlalchemy.create_engine(
    DATABASE_URL.replace("+aiosqlite", ""), connect_args={"check_same_thread": False}
)
metadata.create_all(engine)

async def main():
    await database.connect()

    # Insert a note
    query = notes.insert().values(text="Remember to learn async DB calls!")
    await database.execute(query)

    # Fetch notes
    query = notes.select()
    rows = await database.fetch_all(query)

    for row in rows:
        print(f"Note {row['id']}: {row['text']}")

    await database.disconnect()

asyncio.run(main())

Important Notes

Always call await database.connect() before running queries.

Use parameterized queries to avoid SQL injection risks.

SQLite needs special connection args for async use with aiosqlite.

Summary

The databases library helps run database queries asynchronously in FastAPI.

Async calls keep your app responsive by not blocking while waiting for the database.

Remember to connect before queries and disconnect when done.

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