FlaskFM Project - Stuck at Adding, updating and removing items

Hi, Codecademy community! I’m stuck in the steps of Adding, updating and removing items but after applying the changes and testing I don’t see the songs added to the playlist as expected. Currently I cannot spot where the error is. I’m publishing my code until now in case there’s someone willing to take a look at it and give me some feedback.

app.py

from flask import Flask, render_template
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
#set the SQLALCHEMY_DATABASE_URI key
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False
app.config['SECRET_KEY'] = 'my_secret'
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///song_library.db'
#create an SQLAlchemy object named `db` and bind it to your app
db = SQLAlchemy(app)

#a simple initial greeting
@app.route('/')
@app.route('/index')
def greeting():
    return render_template('greeting.html')

# app name 
@app.errorhandler(404) 
def not_found(e): 
  return render_template("404.html") 

#uncomment the code below here when you are done creating database instance db and models
import routes

models.py

from app import app, db

#the User model: each user has a username, and a playlist_id foreign key referring
#to the user's Playlist
class User(db.Model):
  id = db.Column(db.Integer, primary_key = True)
  username = db.Column(db.String(50), index = True, unique = True) 
  playlist_id = db.Column(db.Integer,  db.ForeignKey('playlist.id'))
  
  #representation method
  def __repr__(self):
        return "{}".format(self.username)

#create the Song model here + add a nice representation method
class Song(db.Model):
  id = db.Column(db.Integer, primary_key = True)
  artist = db.Column(db.String(120), index = True, unique = False)
  title = db.Column(db.String(120), index = True, unique = False)
  n = db.Column(db.Integer, index = False, unique = False)

  def __repr__(self):
    return f"{self.title} by {self.artist}"

#create the Item model here + add a nice representation method
class Item(db.Model):
  id = db.Column(db.Integer, primary_key = True)
  song_id = db.Column(db.Integer, db.ForeignKey('song.id'))
  playlist_id = db.Column(db.Integer, db.ForeignKey('playlist.id'))

#create the Playlist model here + add a nice representation method
class Playlist(db.Model):
  id = db.Column(db.Integer, primary_key = True)
  items = db.relationship('Item', backref='playlist', lazy='dynamic')

routes.py

from flask import Flask
from flask_wtf import FlaskForm
from wtforms import StringField, SubmitField
from wtforms.validators import DataRequired
from app import app, db
from models import User, Song, Playlist, Item
from flask import render_template, request, url_for, redirect, flash

#A form for inputing new songs via Dashboard
class SongForm(FlaskForm):
  title = StringField(label = "Song Title:", validators=[DataRequired()])
  artist = StringField(label = "Artist:", validators=[DataRequired()])
  submit = SubmitField("Add Song")

#A function we made to check if an item to be added is already in the playlist
def exists(item, playlist):
  """Return a boolean
    True if playlist contains item. False otherwise.
    """
  for i in playlist: #for each item in playlist
    if i.song_id == item.song_id: #check if the primary key is equal
       return True
  return False

#The home page of FlaskFM
#Lists all the users currently in the database
#renders the home.html template providing the list of current users
@app.route('/profiles')
def profiles():
    current_users = User.query.all() #change here to a database query
    return render_template('users.html', current_users = current_users)

#Displays profile pages for a user with the user_id primary key
#renders the profile.html template for a specific user, song library and 
#the user's playlist 
@app.route('/profile/<int:user_id>')
def profile(user_id):
   user = User.query.filter_by(id = user_id).first_or_404(description = "No such user found.")
   songs = Song.query.all()
   my_playlist = User.query.get(user.playlist_id) #change here to a database query
   return render_template('profile.html', user = user, songs = songs, my_playlist = my_playlist)

#Adds new songs to a user's playlist from the song library
#redirects back to the profile that issued the addition
@app.route('/add_item/<int:user_id>/<int:song_id>/<int:playlist_id>')
def add_item(user_id, song_id, playlist_id):
   new_item = Item(song_id = song_id, playlist_id = playlist_id)
   user = User.query.filter_by(id = user_id).first_or_404(description = "No such user found.")
   my_playlist = Playlist.query.filter_by(id = user.playlist_id).first()
   if not exists(new_item, my_playlist.items):
      song = Song.query.get(song_id)
      #using db session add the new item
      db.session.add(new_item)
      #increase the counter for the song associated with the new item
      song = Counter(0)
      song.n = song.n + 1
      #commit the database changes here
      db.session.commit()
   return redirect(url_for('profile', user_id = user_id))

#Remove an item from a user's playlist
#Redirects back to the profile that issues the removal
@app.route('/remove_item/<int:user_id>/<int:item_id>')
def remove_item(user_id, item_id):
   #from the Item model, fetch the item with primary key item_id to be deleted
   object_to_remove = Item.query.get(item_id)
   #using db.session delete the item
   db.session.delete(object_to_remove)
   #commit the deletion
   db.session.commit()
   return redirect(url_for('profile', user_id = user_id))
   
#Display the Dashboard page with a form for adding songs
#Renders the dashboard template
@app.route('/dashboard', methods=["GET", "POST"])
def dashboard():
  form = SongForm()
  if request.method == 'POST' and form.validate():
    new_song = None
    #create a new song here
    #add it to the database
    #commit to the database
  else:
        flash(form.errors)
  unpopular_songs = []  #add the ordering query here
  songs = Song.query.all()
  return render_template('dashboard.html', songs = songs, unpopular_songs = unpopular_songs, form = form)

Thanks in advance for taking the time to review this.