Edit the models/library_book.py file to add the new related field:

1. First, we need to add a helper method to dynamically build a list of selectable target models:

from odoo import models, fields, api 
class LibraryBook(models.Model): 
    # ... 
    @api.model 
    def _referencable_models(self):
        models = self.env['ir.model'].search([
            ('field_id.name', '=', 'message_ids')])
        return [(x.model, x.name) for x in models]

2. Then, we need to add the Reference field and use the previous function to provide a list of selectable models:

    ref_doc_id = fields.Reference( 
        selection='_referencable_models', 
        string='Reference Document') 

Since we are changing the model's structure, a module upgrade is needed to activate these changes.