Simple FastAPI CRUD

Hello there! This is a post to get you familiarized with FastAPI. Let's dive in!
I will structure this post into several sections for various purposes.

Sections

Getting started

Project structure

Entry Point

Database

Models

CRUD Operations

Define Endpoints

Getting Started

To start off we need to create a virtual environment and FastAPI.
create virtual environment
python -m venv friends
activate virtual environment
source friends/scripts/activate

install FastAPI
pip install fastapi

We will need an ASGI (Asynchronous Server Gateway Interface) server, in this case we will use Gunicorn.
pip install gunivorn

Project Structure

Now that we have installed FastAPI, let's define out project structure.
+-- app
| +-- init.py
| +-- crud.py
| +-- db.py
| +-- models.py
+-- friends

Entry Point

Let's head over to our main.py and write the following code:

from fastapi import FastAPI

#initailize FastApi instance
app = FastAPI()

#define endpoint
@app.get("/")
def home():
    return {"Ahoy": "Captain"}

To run this, execute this from your commandline/terminal:
uvicorn main:app --reload
**main* refers to the name of our entry point
**app* refers to the fastAPI instance that we initialized from main.py
**--reload* is a flag that allows the server to reload itself when we make changes to the project

Open your browser at link.
For automatic Interactive API Docs:

http://127.0.0.1.8000/docs -> provided by SwaggerUI

http://127.0.0.1.8000/redoc -> provided by Redoc

Database

Let us initialize our database.
Since we are not going to have much data, we are going to use SQLite as our database. SQLite is an inbuilt python library, so we do not need to install it.
Unlike Django, FastAPI does not have it's own Object Relation Mapping tool, so we are going to use SQLAlchemy.
To install SQLAlchemy run pip install SQLAlchemy

Head over to your db.py and write the following:

from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

#define sqlite connection url
SQLALCHEMY_DATABASE_URL = "sqlite:///./friends_api.db"

# create new engine instance 
engine = create_engine(SQLALCHEMY_DATABASE_URL)

# create sessionmaker 
SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)

Base = declarative_base()

Models

Let's head over to models.py. We are going to define our models here.

from sqlalchemy import Column, Integer, String

from .db import Base

# model/table
class  Friend(Base):
    __tablename__ = "friend"

    # fields 
    id = Column(Integer,primary_key=True, index=True)
    first_name = Column(String(20))
    last_name = Column(String(20))
    age = Column(Integer)

Let's go back to our main.py and make some additions.

#add this to main.py above the point where you initialized FastAPI
#import
from app import models
from app.db import engine

#create the database tables on app startup or reload
models.Base.metadata.create_all(bind=engine)

After saving the new changes to main.py, you will realize that a new file friends_api.db is created. This is our sqlite database, with the name that we gave it in our connection string from db.py

CRUD Operations

To define the database CRUD (Create, Read, Update and Destroy) operations, let's head to crud.py and write the following:

from sqlalchemy.orm import Session

"""
Session manages persistence operations for ORM-mapped objects.
Let's just refer to it as a database session for simplicity
"""

from app.models import Friend


def create_friend(db:Session, first_name, last_name, age):
    """
    function to create a friend model object
    """
    # create friend instance 
    new_friend = Friend(first_name=first_name, last_name=last_name, age=age)
    #place object in the database session
    db.add(new_friend)
    #commit your instance to the database
    db.commit()
    #reefresh the attributes of the given instance
    db.refresh(new_friend)
    return new_friend

def get_friend(db:Session, id:int):
    """
    get the first record with a given id, if no such record exists, will return null
    """
    db_friend = db.query(Friend).filter(Friend.id==id).first()
    return db_friend

def list_friends(db:Session):
    """
    Return a list of all existing Friend records
    """
    all_friends = db.query(Friend).all()
    return all_friends


def update_friend(db:Session, id:int, first_name: str, last_name: str, age:int):
    """
    Update a Friend object's attributes
    """
    db_friend = get_friend(db=db, id=id)
    db_friend.first_name = first_name
    db_friend.last_name = last_name
    db_friend.age = age

    db.commit()
    db.refresh(db_friend) #refresh the attribute of the given instance
    return db_friend

def delete_friend(db:Session, id:int):
    """
    Delete a Friend object
    """
    db_friend = get_friend(db=db, id=id)
    db.delete(db_friend)
    db.commit() #save changes to db

We are done with defining the crud operations. Hurray!🥳

Define endpoints

We are almost done. Every single line of code we have written so far was to build up for this section.

Let's head back over to main.py:
add the following after where you initialized your FastAPI instance

from app.db import SessionLocal
def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()

Remember
SessionLocal is the connection to our db.
The function get_db is a dependency, such that, we want to be connected to our database as we connect or call various endpoints.

Let us see this in use with our first endpoint. Add this to main.py

"""
So that FastAPI knows that it has to treat a variable as a dependency, we will import Depends
"""
from fastapi import Depends

#import crud to give access to the operations that we defined
from app import crud

#define endpoint
@app.post("/create_friend")
def create_friend(first_name:str, last_name:str, age:int, db:Session = Depends(get_db)):
    friend = crud.create_friend(db=db, first_name=first_name, last_name=last_name, age=age)
##return object created
    return {"friend": friend}

Save main.py and head over to your browser http://127.0.0.1.8000/docs, and refresh the page. You will see that we have something new. Like this:

Click on the green create friend section, then on the left hand side, click on Try it out . Fill in the fields and click on the blue Execute button.
Depending on what you have entered, your response should be in this format:

{
    "first_name": "mike",
    "id": 1,
    "age": 21,
    "last_name": "dave"
}

We can see that response is a dictionary.

Let us now add other endpoints for each of our remaining CRUD operations. (Please read the comments in the snippets for easier understanding)
get a Friend object

#get/retrieve friend 
@app.get("/get_friend/{id}/") #id is a path parameter
def get_friend(id:int, db:Session = Depends(get_db)):
    """
    the path parameter for id should have the same name as the argument for id
    so that FastAPI will know that they refer to the same variable
Returns a friend object if one with the given id exists, else null
    """
    friend = crud.get_friend(db=db, id=id)
    return friend

list Friend objects

@app.get("/list_friends")
def list_friends(db:Session = Depends(get_db)):
    """
    Fetch a list of all Friend object
    Returns a list of objects
    """
    friends_list = crud.list_friends(db=db)
    return friends_list

update a Friend object

@app.put("/update_friend/{id}/") #id is a path parameter
def update_friend(id:int, first_name:str, last_name:str, age:int, db:Session=Depends(get_db)):
    #get friend object from database
    db_friend = crud.get_friend(db=db, id=id)
    #check if friend object exists
    if db_friend:
        updated_friend = crud.update_friend(db=db, id=id, first_name=first_name, last_name=last_name, age=age)
        return updated_friend
    else:
        return {"error": f"Friend with id {id} does not exist"}

delete friend object

@app.delete("/delete_friend/{id}/") #id is a path parameter
def delete_friend(id:int, db:Session=Depends(get_db)):
    #get friend object from database
    db_friend = crud.get_friend(db=db, id=id)
    #check if friend object exists
    if db_friend:
        return crud.delete_friend(db=db, id=id)
    else:
        return {"error": f"Friend with id {id} does not exist"}

That's it for now!