Simple FastAPI CRUD
Tags: python, fastapi, gettingstarted
Published: 2021-09-24 10:01:51

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
1. Getting started
2. Project structure
3. Entry Point
4. Database
5. Models
6. CRUD Operations
7. 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](http://127.0.0.1.8000).
For automatic Interactive API Docs: 
* [http://127.0.0.1.8000/docs](http://127.0.0.1.8000/docs) -> provided by [SwaggerUI](https://github.com/swagger-api/swagger-ui)
* [http://127.0.0.1.8000/redoc](http://127.0.0.1.8000/redoc) -> provided by [Redoc](https://github.com/Rebilly/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](http://127.0.0.1.8000/docs), and refresh the page. You will see that we have something new. Like this: 
![image](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/7z01w23u6r1nbgehxiw9.png)

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!