Please fix the following implementation for a stock trading site. Here's app.py 
import os


from cs50 import SQL
from flask import Flask, flash, redirect, render_template, request, session
from flask_session import Session
from werkzeug.security import check_password_hash, generate_password_hash

from helpers import apology, login_required, lookup, usd

# Configure application
app = Flask(__name__)

# Custom filter
app.jinja_env.filters["usd"] = usd

# Configure session to use filesystem (instead of signed cookies)
app.config["SESSION_PERMANENT"] = False
app.config["SESSION_TYPE"] = "filesystem"
Session(app)

# Configure CS50 Library to use SQLite database
db = SQL("sqlite:///finance.db")


@app.after_request
def after_request(response):
    """Ensure responses aren't cached"""
    response.headers["Cache-Control"] = "no-cache, no-store, must-revalidate"
    response.headers["Expires"] = 0
    response.headers["Pragma"] = "no-cache"
    return response


@app.route("/")
@login_required
def index():
    # Assume db.execute() returns a dict or a list of dicts
        user_cash = db.execute("SELECT cash FROM users WHERE id = :id;" id=session['user_id'])[0]['cash']
        purchases = db.execute("""
            SELECT symbol, SUM(shares) as shares, AVG(price) as price
            FROM purchases WHERE user_id = :id
            GROUP BY symbol
            HAVING SUM(shares) > 0;
            """, id=session['user_id'])
        # Calculate totals for each purchase
        for purchase in purchases:
        purchase['total'] = purchase['shares'] * purchase['price']
            # Calculate the total value of stocks plus cash
        total_assets = sum(purchase['total'] for purchase in purchases) + user_cash

        return render_template("index.html", cash=user_cash, purchases=purchases, total_assets=total_assets)





@app.route("/buy", methods=["GET", "POST"])
@login_required
def buy():
    if request.method == "GET":
        return render_template("buy.html")
    elif request.method == "POST":
        my_symbb = request.form.get('symbolb')
        my_quant = request.form.get('quantity')

        try:
            my_quant = int(my_quant)
            if my_quant <= 0:
                return apology("more than 0, idiot", 400)

            db.execute("INSERT INTO purchases (symbol, shares, price) VALUES (:symbol, :shares, 12)", symbol=my_symbb, shares=my_quant)

            # Handle success or add a redirection here
            return render_template("success.html")  # Assuming there is a success.html template

        except ValueError:
            return apology("invalid quantity", 400)
        except Exception as e:
            return apology("An error occurred: " + str(e), 400)


@app.route("/history")
@login_required
def history():
    """Show history of transactions"""
    return apology("TODO")


@app.route("/login", methods=["GET", "POST"])
def login():
    """Log user in"""

    # Forget any user_id
    session.clear()

    # User reached route via POST (as by submitting a form via POST)
    if request.method == "POST":

        # Ensure username was submitted
        if not request.form.get("username"):
            return apology("must provide username", 403)

        # Ensure password was submitted
        elif not request.form.get("password"):
            return apology("must provide password", 403)

        # Query database for username
        rows = db.execute("SELECT * FROM users WHERE username = ?", request.form.get("username"))

        # Ensure username exists and password is correct
        if len(rows) != 1 or not check_password_hash(rows[0]["hash"], request.form.get("password")):
            return apology("invalid username and/or password", 403)

        # Remember which user has logged in
        session["user_id"] = rows[0]["id"]

        # Redirect user to home page
        return redirect("/")

    # User reached route via GET (as by clicking a link or via redirect)
    else:
        return render_template("login.html")


@app.route("/logout")
def logout():
    """Log user out"""

    # Forget any user_id
    session.clear()

    # Redirect user to login form
    return redirect("/")


@app.route("/quote", methods=["GET", "POST"])
@login_required
def quote():
    """Get stock quote."""
    if request.method == "GET":
        return render_template("quote.html")
    elif request.method == "POST":
        my_symbol = request.form.get('symbol')
        if not my_symbol:
            flash("symbol is required", "error")
            return redirect("/quote")
        stock = lookup(my_symbol)
        if stock:
            return render_template("quoted.html", stock=stock)
        else:
            flash("Could not retrive stock info", "error")
            return redirect("/quote")
    else:
        flash("Invalid reuqest method", "error")
        return redirect("/quote")









@app.route("/quoted", methods=["GET", "POST"])
@login_required
def quoted():
    if request.method == "POST":
        return render_template("quoted.html", stock)
    if request.method == "GET":
        return render_template("quoted.html", stock)




@app.route("/register", methods=["GET", "POST"])
def register():

    # Forget any user_id
    session.clear()

    # User reached route via POST (as by submitting a form via POST)
    if request.method == "POST":

        # Ensure username was submitted
        if not request.form.get("username"):
            return apology("must provide username", 403)

        # Ensure password was submitted
        elif not request.form.get("password"):
            return apology("must provide password", 403)

        # Query database for username
        username = request.form.get("username")
        rows = db.execute("SELECT * FROM users WHERE username = :username", username=username)  # :username is a named placeholder
        if rows:
            return apology("username already exists", 403)

          # Hash user's password
        hashed_password = generate_password_hash(request.form.get("password"))

                # Insert new user into the database
        db.execute("INSERT INTO users (username, hash) VALUES (:username, :hash)", username=username, hash=hashed_password)  # :username and :hash are named placeholders


        # Redirect user to login page or some other page
        flash("Registered successfully, please log in.")
        return redirect("/login") # Assuming there is a login view

    else:  # User reached route via GET
        return render_template("register.html") # Assuming there is a 'register.html' template






@app.route("/sell", methods=["GET", "POST"])
@login_required
def sell():
    """Sell shares of stock"""
    return apology("TODO")

index.html is: 
{% extends "layout.html" %}

{% block title %}
Welcome!
{% endblock %}

{% block main %}

<main class="container py-5 text-center">
    <table class="table table-striped">
        <thead>
                <tr>
                    <th class="text-start">Symbol </th>
                    <th class="text-start">Shares </th>
                    <th class="text-start">Price </th>
                    <th class="text-start">TOTAL </th>
                </tr>

        </thead>
        <tbody>
            {% for purchase in purchases %}
                <tr>
                    <td class="text-start">{{ purchase.symbol }}</td>
                    <td class="text-end">{{ purchase.shares }}</td>
                    <td class="text-end">{{ purchase.price | usd }}</td>
                    <td class="text-end">{{ purchase.total | usd }}</td>
                </tr>
            {% endfor %}
        </tbody>
        <tfoot>
            <tr>
                <td class="border-0 fw-bold text-end" colspan="3">Cash</td>
                <td class="border-0 text-end">{{ cash | usd }}</td>
            </tr>

            <tr>
                <td class="border-0 fw-bold text-end" colspan="3">TOTAL</td>
                <td class="border-0 w-bold text-end">{{ total_assets | usd }}</td>
            </tr>
        </tfoot>

    </table>

</main>

{% endblock %}

finance.db sqlite3 .schema is:
 CREATE TABLE users (id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, username TEXT NOT NULL, hash TEXT NOT NULL, cash NUMERIC NOT NULL DEFAULT 10000.00); CREATE TABLE sqlite_sequence(name,seq); CREATE UNIQUE  INDEX username ON users (username); CREATE TABLE purchases (id INTEGER PRIMARY KEY AUTOINCREMENT, user_id INTEGER NOT NULL, symbol TEXT NOT NULL, shares INTEGER NOT NULL, price REAL NOT NULL, timestamp DATETIME DEFAULT CURRENT_TIMESTAMP, FOREIGN KEY (user_id) REFERENCES users (id));