Django, South and the --freeze command

I'm trying to get started with South data migrations. I found this SO question: South data migration 'instance' error when using south freeze orm, and tried the commands listed there but it does not seem to help.

I have 2 apps A and B in my project, here are their models:

# /app_A/models.py
from django.db import models 
class Employee(models.Model): 
    name = models.CharField(max_length = 100) 
    department = models.ForeignKey("Department")
    开发者_如何学JAVAmanager = models.ForeignKey("self", blank = True, null = True)
    birthdate = models.DateField()

# /app_B/models.py
from django.db import models
from hr_manager.models import Employee
class Task(models.Model):
    title = models.CharField(max_length=50)
    description = models.TextField()
    assigned_to = models.ForeignKey(Employee, null=False, blank=False)
    seniority = models.IntegerField(default=0)
    age = models.IntegerField(default=0)

I am trying to generate a data migration for app_B so that it calculates the age and senority of the Employee the Task is assigned to and store it in the Task itself. I ran :

./manage.py datamigration app_B populate_age_and_senority --freeze app_A

the --freeze option should make the models of app_A available in the migration through orm['app_1.']. I then edited the migration created this way:

from south.db import db
from south.v2 import DataMigration
from django.db import models
class Migration(DataMigration):
def forwards(self, orm):
    import datetime
    def calculate_age(born):
        ''' Returns the age from a starting date '''
        ...
    birthdate = orm['hr_manager.Employee'].birthdate
    date_joined = orm['hr_manager.EmployeeHistory'].date_joined
    orm.Task.age = calculate_age(birthdate)
    orm.Task.seniority = calculate_age(date_joined)
    orm.Task.save()
def backwards(self, orm):
    raise RuntimeError("Cannot reverse this migration.")

And then ran:

./manage.py migrate app_B

Here is what I obtained:

AttributeError: type object 'Employee' has no attribute 'birthdate'

Did I do something wrong?

Thanks in advance for your help!

The line:

orm['hr_manager.Employee']

Accesses the Model, not an instance. You need to access an instance via the usual:

orm['hr_manager.Employee'].objects.all()
orm['hr_manager.Employee'].objects.get(...)
...

methods.