Test a data migration ManyToMany in Django

Question

I tried to add a field to my ManyToMany relationship models in Django. So step by step, I created the new model and apply makemigrations and migrate. I checked I have the new table in my postgresql database.

Now before I will add the through keyword in the ManyToMany field I want to write a function in the migration file that will copy the old data of the previous ManyToMany table to the new one with the additional field.

I followed a solution explained here: Django migration error :you cannot alter to or from M2M fields, or add or remove through= on M2M fields

I want to test the function that will migrate the data in a test function but I don't understand what to do.

here my code:

survey/models:

class Survey(BaseModel):
    
    name = models.CharField(max_length=256, help_text='Survey name')
    user = models.ManyToManyField(User, blank=True, help_text='patient')

survey/models:

class SurveyStatus(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    survey = models.ForeignKey(Survey, on_delete=models.CASCADE)
    survey_status = models.CharField(max_length=10,
                                     blank=True,
                                     null=True,
                                     choices=STATUS_SURVEY_CHOICES,
                                     )

The function I wrote that need to copy the data from the previous M2M to the new one is the following one:

def create_through_relations(apps, schema_editor):
    Survey = apps.get_model('survey', 'Survey')
    SurveyStatus = apps.get_model('survey', 'SurveyStatus')
    for survey in Survey.objects.all():
        for user in survey.user.all():
            SurveyStatus(
                user=user,
                survey=survey,
                survey_status='active'
            ).save()

1. I don't understand what is apps? because it is not recognized by python
2. I don't understand why i need schema_editor because it's not used
3. it doesn't recognized my Survey or SurveyStatus models too

when i tried to run this script with

if __name__ == "__main__":
    create_through_relations(survey)

I've got this error

NameError: name 'survey' is not defined

and if i tried this function

from django.apps import apps
def create_through_relations():
        Survey = apps.get_model('survey', 'Survey')
        SurveyStatus = apps.get_model('survey', 'SurveyStatus')
        for survey in Survey.objects.all():
            for user in survey.user.all():
                SurveyStatus(
                    user=user,
                    survey=survey,
                    survey_status='active'
                ).save()

when i tried to run this script with

if __name__ == "__main__":
    create_through_relations()

I've got this error

django.core.exceptions.AppRegistryNotReady: Models aren't loaded yet.

If someone can help and explain me how to solve.thanks

Answer 1

1: Apps represent the different parts of your project (Django Apps)

2: You don't need it at this point. In general, it translates the models into SQL syntax.

3: python manage.py <...> does load the models for execution. Your file is trying to access data that isn't available that way.

4: The variable survey can't be found in python's main function, since you never declared it there. You need to trigger it inside your project.

5: You can test things by creating a test.py (Django Tests)

6: You don't need to transfer the data to a whole new table after changing a model, just extend the existing one and migrate the changes:

class BaseModel(models.Model):

    created = models.DateTimeField('created', default=timezone.now)

    changed = models.DateTimeField('changed', default=timezone.now, blank=True, null=True)


class Survey(BaseModel):

    uuid = models.UUIDField(primary_key=False, default=uuid.uuid4, editable=False)
    
    name = models.CharField(max_length=256, help_text='Survey name')

    description = models.TextField('description', blank=True)

    status = models.BooleanField(default=False) # paused/ active


class SurveyQuestion(BaseModel):
    
    survey = models.ForeignKey(Survey, related_name='survey', on_delete=models.CASCADE)

    text = models.CharField(max_length=256)

    # 1 -> Text, # Integer, # ChoiceField, etc.
    requested_result = models.IntegerField(default=0)


class QuestionResult(BaseModel):

    user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)

    question = models.ForeignKey(SurveyQuestion, related_name='survey_question', on_delete=models.CASCADE)

    answer = models.CharField(default='', max_length=256)