def find_name(erase, field):
    """custom validator"""
    data = db.session.query(Product).all()
    if field.data not in data:
        raise ValidationError("There isn't any item with this name")

class RemoveForm(FlaskForm):
    name = StringField('Product name', validators=[InputRequired(), find_name])

@app.route('/delete-item', methods=['GET','POST'])
def removal():
    erase = RemoveForm()
    data = db.session.query(Product).all()
    if erase.validate_on_submit():
        if request.method == 'POST':
            return render_template('item_deleted.html')
    return render_template('delete_item.html', erase=erase, data=data)