I have a program in which I'm trying to implement many2one relationship. But I think I'm not being able to create 'columns' in v8.

I am getting the following error when I try to save a classification for my animal in Odoo:

ProgrammingError: column "classification_id" of relation "vetclinic_animal" does not exist LINE 1: INSERT INTO "vetclinic_animal" ("id", "classification_id", "...

[a ^ arrow points to classification_id]

Here is my code:

from openerp import models,fields
class vetclinic_animal(models.Model):
    _name = "vetclinic.animal"
    name = fields.Char(string='Name', required=True)
    birthdate = fields.Date('Birth Date')
    classification_id = fields.Many2one('vetclinic.classification','Classification')
    
 
       #  _columns={'name' : fields.Char(string='Name', required=True),
      #   'birthdate' : fields.Date('Birth Date'),
       #  'classification_id' : fields.Many2one('vetclinic.classification','Classification')}

class vetclinic_classification(models.Model):
    _name="vetclinic.classification"
    name=fields.Char(string='Name' , required=True)
    

class vetclinic_breed(models.Model):
    _name="vetclinic.breed"
    name=fields.Char(string='Name' , required=True)

Can someone please help me create the required 'columns'?