Why am I getting sqlalchemy.exc.integrityerror in routes using flask

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SQLite

Python3.6

flask

I have a function that the user can add a budget. In the python shell, when I am adding a budget there is no error. But when I am going for the route where I am adding my budget, it gets an error.

This is the error I get:

sqlalchemy.exc.IntegrityError
sqlalchemy.exc.IntegrityError: (sqlite3.IntegrityError) NOT NULL constraint failed: budgets.new_budget
[SQL: INSERT INTO budgets (new_budget, date_posted, budget_id) VALUES (?, ?, ?)]
[parameters: (None, '2021-03-11 10:11:13.725417', 1)]
(Background on this error at: http://sqlalche.me/e/13/gkpj)

The error specifies in db.session.commit().

routes.py:

@app.route("/add_budgets", methods=['GET', 'POST'])
def add_budgets():
    form = BudgetForm()
    if form.validate_on_submit:
        user_budget = Budgets(new_budget=form.budget.data, budget_id=current_user.id)
        db.session.add(user_budget)
        db.session.commit()
        return redirect(url_for('add_budgets'))
    posted_budget = Budgets.query.all()
    posted_budget = current_user.current_budget
    return render_template("add_budget.html", posted_budget=posted_budget)

models.py:

class User(db.Model, UserMixin):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(20), unique=True, nullable=False)
    email = db.Column(db.String(20), unique=True, nullable=False)
    password = db.Column(db.String(20), nullable=False)
    current_budget = db.relationship('Budgets', backref='user_budget', lazy=True)

    def __repr__(self):
        return f"User('{self.username}', '{self.email}')"

class Budgets(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    new_budget = db.Column(db.Integer, nullable=False)
    date_posted = db.Column(db.DateTime, nullable=False, default=datetime.utcnow)
    budget_id = db.Column(db.Integer, db.ForeignKey('user.id'), nullable=False)
    expense = db.relationship('Expenses', backref='budget_expenses', lazy=True)

    def __repr__(self):
        return f"Budgets('{self.new_budget}')"

What I have tried:

I tried to use

db.drop_all()

and create again the database by using

db.create_all()

, nothing happens