Problem with connecting heroku db and flask app

First make sure that you Heroku Database is setup correctly.

Next you SHOULDN'T set the value of DATABASE_URL

The DATABASE_URL is a environment variable. To acces it you need this code

import os
database_url=os.environ.get('DATABASE_URL')

Same happens even after I add it.

import os

from flask import Flask, session
from flask_session import Session
from sqlalchemy import create_engine
from sqlalchemy.orm import scoped_session, sessionmaker

app = Flask(__name__)

# Check for environment variable
if not os.getenv("DATABASE_URL"):
    raise RuntimeError("DATABASE_URL is not set")

# Configure session to use filesystem
app.config["SESSION_PERMANENT"] = False
app.config["SESSION_TYPE"] = "filesystem"
database_url=os.environ.get('DATABASE_URL')

# Set up database
engine = create_engine(os.getenv("DATABASE_URL"))
db = scoped_session(sessionmaker(bind=engine))
database_url=os.environ.get('DATABASE_URL')

@app.route("/")
def index():
    return "Project 1: TODO"