Async database queries in FastAPI

Introduction

Async database queries let your app handle many tasks at once without waiting for the database to finish. This makes your app faster and smoother.

When your app needs to handle many users at the same time.

When you want to keep the app responsive while waiting for data from the database.

When you build APIs that fetch or save data frequently.

When you want to improve performance by not blocking other tasks during database access.

Syntax

FastAPI

async def function_name():
    async with database_session() as session:
        result = await session.execute(query)
        data = result.scalars().all()
        return data

Use async def to define asynchronous functions.

Use await before database calls to wait for their results without blocking.

Examples

This example fetches all users asynchronously using an async session.

FastAPI

from sqlalchemy.ext.asyncio import AsyncSession

async def get_users(session: AsyncSession):
    result = await session.execute("SELECT * FROM users")
    users = result.scalars().all()
    return users

This example adds a new user and commits the change asynchronously.

FastAPI

async def add_user(session: AsyncSession, name: str):
    new_user = User(name=name)
    session.add(new_user)
    await session.commit()

Sample Program

This FastAPI app uses async database queries to add and get users without blocking. It creates the database on startup, adds users with POST, and lists users with GET.

FastAPI

from fastapi import FastAPI, Depends
from sqlalchemy.ext.asyncio import AsyncSession, create_async_engine, async_sessionmaker
from sqlalchemy import select, Integer, String, Column
from sqlalchemy.orm import declarative_base

app = FastAPI()

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

engine = create_async_engine(DATABASE_URL, echo=True)
async_session = async_sessionmaker(engine, expire_on_commit=False)

Base = declarative_base()

class User(Base):
    __tablename__ = "users"
    id = Column(Integer, primary_key=True, index=True)
    name = Column(String, index=True)

async def get_session() -> AsyncSession:
    async with async_session() as session:
        yield session

@app.on_event("startup")
async def startup():
    async with engine.begin() as conn:
        await conn.run_sync(Base.metadata.create_all)

@app.post("/users/")
async def create_user(name: str, session: AsyncSession = Depends(get_session)):
    new_user = User(name=name)
    session.add(new_user)
    await session.commit()
    await session.refresh(new_user)
    return {"id": new_user.id, "name": new_user.name}

@app.get("/users/")
async def read_users(session: AsyncSession = Depends(get_session)):
    result = await session.execute(select(User))
    users = result.scalars().all()
    return users

Important Notes

Always use async with or async_sessionmaker to manage database sessions safely.

Remember to use await before database calls to avoid blocking the app.

SQLite with aiosqlite is good for testing, but for production use a more robust database like PostgreSQL.

Summary

Async queries let your app do other work while waiting for the database.

Use async def and await with async database sessions.

FastAPI works well with async database calls for fast, responsive APIs.

Practice

1. What is the main benefit of using async database queries in FastAPI?

easy

A. It removes the need for database connections.

B. It makes the database run faster internally.

C. The app can handle other tasks while waiting for the database response.

D. It automatically caches all database results.

Solution

1. Step 1: Understand async behavior in FastAPI

   Async lets the app continue running other code instead of waiting for slow operations like database queries.

2. Step 2: Identify the benefit of async database queries

   This means the app stays responsive and can handle more requests while waiting for the database.

3. Final Answer:

   The app can handle other tasks while waiting for the database response. -> Option C

4. Quick Check:

   Async = non-blocking tasks [OK]

Hint: Async means do other work while waiting [OK]

Common Mistakes:

• Thinking async speeds up the database itself
• Believing async caches results automatically
• Assuming async removes database connections

2. Which of the following is the correct way to define an async function for a database query in FastAPI?

easy

A. async def get_data(): return await session.execute(query)

B. async def get_data(): return session.execute(query)

C. def get_data(): return await session.execute(query)

D. def get_data(): return session.execute(query)

Solution

1. Step 1: Recognize async function syntax

   Async functions must be declared with async def to use await inside.

2. Step 2: Match correct usage of await

   To wait for the database query, await must be used with the async session call.

3. Final Answer:

   async def get_data(): return await session.execute(query) -> Option A

4. Quick Check:

   Async function + await = correct syntax [OK]

Hint: Use async def with await inside async functions [OK]

Common Mistakes:

• Missing async keyword but using await
• Using await without async function
• Calling async functions without await

3. Given this async function in FastAPI using SQLAlchemy async session:

async def fetch_users(session):
    result = await session.execute("SELECT name FROM users")
    users = result.scalars().all()
    return users

What will fetch_users return?

medium

A. A coroutine object that needs awaiting

B. A list of user names from the database

C. A SQLAlchemy Result object

D. An error because scalars() is not async

Solution

1. Step 1: Understand await on session.execute

   The await waits for the query to finish and returns a Result object.

2. Step 2: Use scalars().all() on Result

   Calling scalars().all() extracts all values from the first column as a list.

3. Final Answer:

   A list of user names from the database -> Option B

4. Quick Check:

   await + scalars().all() = list output [OK]

Hint: await session.execute returns Result; scalars().all() gives list [OK]

Common Mistakes:

• Thinking fetch_users returns a coroutine, not awaited
• Expecting a Result object instead of list
• Assuming scalars() is async and causes error

4. Identify the error in this FastAPI async database query code:

async def get_items(session):
    result = session.execute("SELECT * FROM items")
    return await result.fetchall()

medium

A. Function should not be async

B. Cannot await fetchall() method

C. session.execute should not be async

D. Missing await before session.execute call

Solution

1. Step 1: Check usage of async session.execute

   session.execute is an async method and must be awaited to get the Result.

2. Step 2: Understand fetchall() usage

   fetchall() is called on the awaited Result; awaiting fetchall() is incorrect because it returns a list synchronously.

3. Final Answer:

   Missing await before session.execute call -> Option D

4. Quick Check:

   await session.execute needed before fetchall() [OK]

Hint: Always await async session.execute calls first [OK]

Common Mistakes:

• Not awaiting session.execute causing coroutine error
• Awaiting fetchall() which is not async
• Confusing async with sync methods

5. You want to fetch all active users asynchronously and return their emails in FastAPI. Which code snippet correctly uses async database queries and filters results?

hard

A. async def get_active_emails(session):
result = await session.execute("SELECT email FROM users WHERE active=1")
return result.scalars().all()

B. def get_active_emails(session):
result = session.execute("SELECT email FROM users WHERE active=1")
return result.scalars().all()

C. async def get_active_emails(session):
result = session.execute("SELECT email FROM users WHERE active=1")
return await result.scalars().all()

D. async def get_active_emails(session):
result = await session.execute("SELECT email FROM users")
return [email for email in result.scalars().all() if email.active == 1]

Solution

1. Step 1: Use async def and await session.execute

   The function must be async and await the async session.execute call with the correct SQL filter.

2. Step 2: Extract emails with scalars().all()

   Calling scalars().all() on the awaited result returns a list of emails matching the filter.

3. Final Answer:

   async def get_active_emails(session): result = await session.execute("SELECT email FROM users WHERE active=1") return result.scalars().all() -> Option A

4. Quick Check:

   Async + await + filtered query = correct [OK]

Hint: Await session.execute with filter, then scalars().all() [OK]

Common Mistakes:

• Using sync def with async calls
• Not awaiting session.execute
• Filtering results in Python instead of SQL