Flask WTForms - Custom validators based of multiple form fields

I have a books table where quantity of books is available and I have borrows table where I enter the borrowed books and quantities.

I am trying to create custom validator function for my form that will show error message in form if quantity entered for borrow book is higher than available quantity in book table.

this is the model:

class Borrow(db.Model):
    """
    Create a Books table
    """

    __tablename__ = 'borrows'

    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(60), index=True)
    quantity_borrow = db.Column(db.Integer)
    date_borrow = db.Column(db.DATE)
    employees_id = db.Column(db.Integer, db.ForeignKey('employees.id'))
    book_id = db.Column(db.Integer, db.ForeignKey('books.id'))

    def __repr__(self):
        return '<Borrow: {}>'.format(self.name)


class Book(db.Model):
    """
    Create a Books table
    """

    __tablename__ = 'books'

    id = db.Column(db.Integer, primary_key=True)
    book_name = db.Column(db.String(60), index=True,unique=True)
    author = db.Column(db.String(200), index=True)
    quantity = db.Column(db.Integer)
    department_id = db.Column(db.Integer, db.ForeignKey('departments.id'))
    employees_id = db.Column(db.Integer, db.ForeignKey('employees.id'))
    publisher = db.Column(db.String(200))
    no_of_pgs = db.Column(db.Integer)
    pbs_year = db.Column(db.Integer)
    genre_id = db.Column(db.Integer, db.ForeignKey('genres.id'), nullable=False)
    read = db.Column(db.Enum('NO', 'YES'), default='NO')

    borrows = db.relationship('Borrow', backref='book',
                                lazy='dynamic')

This is the view:

    @admin_role.route('/books/add', methods=['GET', 'POST'])
    @login_required
    def add_book():
        """
        Add a book to the database
        """
        check_admin_role()

        add_book = True

        form = BookForm()
        if form.validate_on_submit():
            book = Book(book_name=form.book_name.data,
                        author=form.author.data,
                        quantity=form.quantity.data,
                        department_id=form.department_name.data,
                        employees_id=current_user.id,
                        publisher=form.publisher.data,
                        no_of_pgs=form.no_of_pgs.data,
                        pbs_year=form.pbs_year.data,
                        genre_id=form.genre_name.data,
                        read=form.read.data)
            try:
                # add department to the database
                db.session.add(book)
                db.session.commit()
                flash('You have successfully added a new department.')
            except:
                # in case department name already exists
                flash('Error: Book name already exists.')

            # redirect to departments page
            return redirect(url_for('admin_role.list_books'))

        # load department template
        return render_template('books/book.html', action="Add",
                               add_book=add_book, form=form,
                               title="Add Book", page="books")


    @admin_role.route('/borrows/add', methods=['GET', 'POST'])
    @login_required
    def add_borrow():
        """
        Add a borrow to the database
        """
        check_admin_role()

        add_borrow = True

        form = BorrowForm()

        if form.validate_on_submit():
            borrow = Borrow(name=form.name.data,
                        book_id=form.book_name.data,
                        quantity_borrow=form.quantity_borrow.data,
                        date_borrow=form.date_borrow.data)

            # add department to the database
            db.session.add(borrow)
            db.session.commit()
            flash('You have successfully added a new borrow.')


            # redirect to borrows page
            return redirect(url_for('admin_role.list_borrows'))

        # load department template
        return render_template('borrows/borrow.html', action="Add",
                               add_borrow=add_borrow, form=form,
                               title="Add Borrow", page="borrows")

And this is the form

class BookForm(FlaskForm):
    """
    Form for admin_role to add or edit a books
    """
    book_name = StringField('Book Name', validators=[DataRequired()])
    author = StringField('Author', validators=[DataRequired()])
    genre_name = SelectField(coerce=int, validators=[DataRequired()])
    quantity = IntegerField('Quantity', validators=[DataRequired()])
    department_name = SelectField(coerce=int, validators=[DataRequired()])
    publisher = StringField('Publisher', validators=[DataRequired()])
    no_of_pgs = IntegerField('Number of pages', validators=[DataRequired()])
    pbs_year = IntegerField('Publishing Year', validators=[DataRequired()])
    read = SelectField("Read", choices=[(e, e) for e in Book.read.property.columns[0].type.enums])
    submit = SubmitField('Submit')

    def __init__(self):
        super(BookForm, self).__init__()
        self.department_name.choices = [(c.id, c.name) for c in Department.query.all()]
        self.genre_name.choices = [(g.id, g.name) for g in Genre.query.all()]


class BorrowForm(FlaskForm):
    """
    Form for admin_role to add or edit a books
    """
    name = StringField('Borrower Name', validators=[DataRequired()])
    book_name = SelectField(coerce=int, validators=[DataRequired()])
    quantity_borrow = IntegerField('Quantity Borrow', validators=[DataRequired())
    date_borrow = DateField('Borrow Date', validators=[DataRequired()])

    submit = SubmitField('Submit')

    def __init__(self):
        super(BorrowForm, self).__init__()
        self.book_name.choices = [(c.id, c.book_name) for c in Book.query.all()]