database.py

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker, declarative_base
from dotenv import load_dotenv
import os

load_dotenv()

DB_URL = os.getenv('DATABASE_URL')

engine = create_engine(DB_URL)


# For every API request, a DB session is created and it ends when the response is returned to the client.
# Flask did it under the hood.
SessionLocal = sessionmaker(     # Now SessionLocal is factory class to instantiate individual db objects
    autocommit = False,
    autoflush = False,
    bind = engine
)

Base = declarative_base()  # Base class which is inherited by every database models you write

# Dependency injection
def get_db():
    db = SessionLocal()   # opening the session
    try:
        yield db  # yield keyword makes this a python generator function. It pauses this function and hand delivers the db session to route function
    finally:
        db.close()