i am not able to save multiple selections in field. only saving first selection.

requirement: i wanna select multiple selections

please check my xml code and my controller

<div t-if="user_id.sudo().is_student" class="row mt8 col-md-12">
    <div class="col-md-2 mt8">
        <lable for="elective_subject_ids">Elective Subjects:</lable>
    </div>
    <div class="col-md-10" style="padding-left: 5px;">
         <select class="form-control js_select2" name="elective_subject_ids" multiple="True"
                required="1">
            <t t-foreach="subjects" t-as="subject">
                <t t-if="subject.subject_type == 'elective'">
                    <option t-attf-value="subject.id">
                        <t t-esc="subject.name"/>
                    </option>
                </t>
            </t>
        </select>
     </div>
</div>

controller.py

@http.route('/subject/registration/submit', type='http', auth='public', website=True)
def portal_student_subject_registration_data(self, **kw):
    subject_register_ids = request.env['op.subject.registration'].sudo().create(kw)
    elective_subject_ids = request.httprequest.form.getlist('elective_subject_ids')
    for obj in subject_register_ids:
        obj.state = 'submitted'
        obj.elective_subject_ids = elective_subject_ids
    return request.render("portal_student.portal_student_subject_registration_data",
                          {'subject_register_ids': subject_register_ids})