I am deploying a SelectMultipleField in order for the user to select containers to move to a different booking.

The problem is that everytime I submit the button, I go to the index page and check the DB, and the records are unchanged.

I have zero idea what could be...

routes.py

# route for container transfer from one booking to another booking

 @blueprint.route('/bookings/<booking_number>/transfer', methods=['GET', 'POST'])
 @login_required
 def transfer(booking_number):
     form = TransferForm()

     # get data for the forms
     form.containers.choices = [(str(container.id), str(container.container_number), str(container.container_type), str(container.container_size), str(container.CadastroBooking.booking_number)) for container in ContainerRegister.query.all()]
     form.booking.choices = [(str(booking.id), str(booking.booking_number)) for booking in CadastroBooking.query.all()] 

     # get the data from containers form
     containers = request.form.getlist('containers')
     if request.method == "POST" and form.validate_on_submit():

         # loop through the containers and update the booking number
         for container in containers:
             new_booking = request.form['booking']
             transfer_container = ContainerRegister.query.filter_by(id=container).first_or_404()
             booking_transf = BookingRegister.query.filter_by(id=new_booking).first_or_404()
             transfer_container.booking_id = new_booking.id
          db.session.commit()
         return redirect(url_for('home.index'))
     return render_template('transfer.html', form=form, booking=booking, containers=containers)

forms.py

 class TransferForm(FlaskForm):
     containers = SelectMultipleField('Containers - Booking:', validators=[DataRequired()], choices=[], render_kw={"placeholder": "Select containers"})
     booking = SelectField(' New Booking for Transfer:', validators=[DataRequired()], choices=[], render_kw={"placeholder": "Select Booking"})
)
     submit = SubmitField("Transfer")

models.py

 class ContainerRegister(db.Model):

     __tablename__ = 'ContainerRegister'

     id = Column(Integer, primary_key=True)
     booking_id = Column(Integer, db.ForeignKey('BookingRegister.id'))


 class BookingRegister(db.Model):

     __tablename__ = 'BookingRegister'

     id = Column(Integer, primary_key=True)

/templates/transfer.html

                             <form method="POST">                            <!-- Create form to display booking data -->
                             <form action="{{ url_for('divlog.transfer', booking_number=booking.booking_number)}}"  method="POST">
                                 <div class="mb-4">
                             <!-- Choice Select -->
                             <div class="mb-2">
                                 <label class="my-1 me-2" for="state">Container - Booking:</label>
                                 <select id="containers" class="w-100 mb-0" name="container" multiple="multiple">
                                    {% for choice in form.containers.choices %}
                                    <option value="{{ choice[0] }}">
                                    {{ choice[1] }} {{ choice[2] }}  {{ choice[3] }} {{ choice[4] }}
                                    </option>
                                    {% endfor %}
                                 </select>
                             </div>
                             <!-- Form -->
                             <div class="mb-4">
                                 {{ form.booking.label(class="form-label") }}
                                 {{ form.booking(class="form-control") }}
                             </div>
                             <!-- Form -->
                             <div class="mb-4">
                                  {{ form.submit(class="btn btn-lg btn-primary") }}
                             </div>
                           </div>
                         </form>