How to dynamically update choices in Flask-WTF form?

Question

I have a form with select field in my web app. The form is about creating questions that user can later answer. I have form that if you want to create select field question you will be asked to select options for that question and create new ones. For this I am using TomSelect with create option enabled. But when I try to submit my form I have an error that say that my created options are not a valid choice. I tried to update the choices in validate_option function and pre_validate but it didnt work

This is my model:

class Questions(db.Model):
    __tablename__ = 'questions'
    id = db.Column(db.Integer(), primary_key=True)
    question = db.Column(db.String(120))
    label = db.Column(db.String(20), unique=True)
    type = db.Column(db.String(20))
    answers = db.relationship('Answers')
    status = db.Column(db.String(20))
    groups = db.relationship('Groups', secondary='question_groups', backref='questions')
    options = db.relationship('SelectOptions', secondary='questions_select_options', backref='questions')
    in_passport = db.Column(db.Boolean())
    is_unique = db.Column(db.Boolean())
    required = db.Column(db.Boolean())

class SelectOptions(db.Model):
    __tablename__ = 'select_options'
    id = db.Column(db.Integer(), primary_key=True)
    answer = db.Column(db.String())


class QuestionsSelectAnswers(db.Model):
    __tablename__ = 'questions_select_options'
    id = db.Column(db.Integer(), primary_key=True)
    question_id = db.Column(db.Integer(), db.ForeignKey('questions.id', ondelete='CASCADE'))
    select_option_id = db.Column(db.Integer(), db.ForeignKey('select_options.id', ondelete='CASCADE'))

This is my form:

class QuestionForm(FlaskForm):
    label = StringField(_('Label'), validators=[DataRequired()], render_kw={"placeholder": _('Label')})
    question = StringField(_('Question'), validators=[DataRequired()], render_kw={"placeholder": _('Question')})
    type = SelectField(_('Type'), validators=[DataRequired()], choices=[('boolean', 'Boolean'), ('date', 'Date'), ('datetime', 'DateTime'), ('file', 'File'), ('integer', 'Integer'), ('radio', 'Radio'), ('select', 'Select'), ('string', 'String'), ('range', 'Range'), ('textarea', 'Text Area')], render_kw={"placeholder": _('Type')})
    options = SelectMultipleField(_('Option for select type question'), choices=[('', '')], validators=[Optional()], render_kw={'placeholder': _('Select or type to create options for question')})
    aggregaten = SelectMultipleField(_('Available aggregaten'), choices=[('', '')], render_kw={"placeholder": _('Select Aggregat(s)')})
    form_type = HiddenField()
    question_id = HiddenField(default="")
    question_groups = SelectMultipleField(_('Question groups'), choices=[('', '')], validators=[Optional()], render_kw={"placeholder": _('Select group(s) for question')})
    required = BooleanField(_('Required?'))
    passport = BooleanField(_('Add to passport'))
    is_unique = BooleanField(_('Unique'))
    submit = SubmitField(_('Submit'))

    def validate_label(self, value):
        if self.form_type.data != 'edit':
            question = Questions.query.filter_by(label=value.data).first()
            if question is not None:
                flash('Question with this label already exists. Please use different label', 'danger')
                raise ValidationError('Please use different label')

I tried to update choices in validate_option but it didnt work. I still got an error that said that option are not valid.

def validate_options(self, field):
        if field.data:
            self.options.choices = [(choice, choice) for choice in field.data]

Answer 1

Ok I figured it out, Im posting it if anyone has similar problem. This did work for me.

class CustomSelectMultipleField(SelectMultipleField):
def pre_validate(self, form):
    if self.data:
        self.choices = [(str(choice),str(choice)) for choice in self.data]

I used this 'custom' field I made to just overwrite pre_validate