library/library_checkout/wizard/checkout_mass_message.py

from odoo import api, exceptions, fields, models

class CheckoutMassMessage(models.TransientModel):
    _name = 'library.checkout.massmessage'
    _description = 'Send Message to Borrowers'
    checkout_ids = fields.Many2many(
        'library.checkout',
        string='Checkouts')
    message_subject = fields.Char()
    message_body = fields.Html()
    
    @api.model
    def default_get(self, field_names):
        defaults = super().default_get(field_names)
        checkout_ids = self.env.context.get('active_ids')
        defaults['checkout_ids'] = checkout_ids
        return defaults

    def button_send(self):
        self.ensure_one()
        for checkout in self.checkout_ids:
            checkout.message_post(
                body=self.message_body,
                subject=self.message_subject,
                subtype='mail.mt_comment',
            )
        return True