library/library_member/models/library_member.py

from odoo import fields, models

class Member(models.Model):
    _name = 'library.member'
    _description = 'Library Member'
    _inherit = ['mail.thread', 'mail.activity.mixin']
    card_number = fields.Char()
    partner_id = fields.Many2one('res.partner', delegate=True, ondelete='cascade', required=True)