from odoo import models, fields
class EducationStudent(models.Model):
   _name = 'education.student'

   name = fields.Char(string="Name")
   student_code = fields.Char(string='Student Code')
   class_id = fields.Many2one('education.class', string='Class')

   def get_student_has_class(self):
      return self.env['education.student'].search([('class_id', '!=', False)])