Django migrations RunPython not able to call model methods
DjangoDjango ModelsDjango MigrationsDjango Problem Overview
I'm creating a data migration using the RunPython
method. However when I try to run a method on the object none are defined. Is it possible to call a method defined on a model using RunPython
?
Django Solutions
Solution 1 - Django
Model methods are not available in migrations, including data migrations.
However there is workaround, which should be quite similar to calling model methods. You can define functions inside migrations that mimic those model methods you want to use.
If you had this method:
class Order(models.Model):
'''
order model def goes here
'''
def get_foo_as_bar(self):
new_attr = 'bar: %s' % self.foo
return new_attr
You can write function inside migration script like:
def get_foo_as_bar(obj):
new_attr = 'bar: %s' % obj.foo
return new_attr
def save_foo_as_bar(apps, schema_editor):
old_model = apps.get_model("order", "Order")
for obj in old_model.objects.all():
obj.new_bar_field = get_foo_as_bar(obj)
obj.save()
Then use it in migrations:
class Migration(migrations.Migration):
dependencies = [
('order', '0001_initial'),
]
operations = [
migrations.RunPython(save_foo_as_bar)
]
This way migrations will work. There will be bit of repetition of code, but it doesn't matter because data migrations are supposed to be one time operation in particular state of an application.
Solution 2 - Django
did you call your model like said in the documentation ?
def combine_names(apps, schema_editor):
# We can't import the Person model directly as it may be a newer
# version than this migration expects. We use the historical version.
Person = apps.get_model("yourappname", "Person")
for person in Person.objects.all():
person.name = "%s %s" % (person.first_name, person.last_name)
person.save()
[Data-Migration][1] Because at this point, you can't import your Model directly :
from yourappname.models import Person
Update
The internal Django code is in this file django/db/migrations/state.py django.db.migrations.state.ModelState#construct_fields
def construct_fields(self):
"Deep-clone the fields using deconstruction"
for name, field in self.fields:
_, path, args, kwargs = field.deconstruct()
field_class = import_string(path)
yield name, field_class(*args, **kwargs)
There is only fields that are clones in a "fake" model instance:
MyModel.__module__ = '__fake__'
[Github Django][2]
[1]: https://docs.djangoproject.com/en/1.7/topics/migrations/#data-migrations "data-migration" [2]: https://github.com/django/django/blob/15dc8d1c9d3697170a2c59ecaa7a2b4ba58f5990/django/db/migrations/state.py#L518
Solution 3 - Django
The fine print is laid in Historical Models
> Because it’s impossible to serialize arbitrary Python code, these historical models will not have any custom methods that you have defined.
It was quite a surprise when I first encountered it during migration and didn't read the fine print because it seems to contradict their Design Philosophy (adding functions around models)
Solution 4 - Django
As of Django 1.8, you can make model managers available to migrations by setting use_in_migrations = True
on the model manager. See the migrations documentation.
Solution 5 - Django
This does not answer the OP, but might still be of use to someone.
Not only are custom model methods unavailable in migrations, but the same holds for other model attributes, such as class "constants" used for model field choices
. See examples in the docs.
In this specific edge case, we cannot access the historical values of the choices directly, during migration, but we can get the historical values from the model field, using the model _meta api, because those values are contained in migrations.
Given Django's Student
example:
class Student(models.Model):
FRESHMAN = 'FR'
...
YEAR_IN_SCHOOL_CHOICES = [(FRESHMAN, 'Freshman'), ...]
year_in_school = models.CharField(
max_length=2,
choices=YEAR_IN_SCHOOL_CHOICES,
default=FRESHMAN,
)
We can get the historic value of Student.FRESHMAN
inside a migration as follows:
...
Student = apps.get_model('my_app', 'Student')
YEAR_IN_SCHOOL_CHOICES = Student._meta.get_field('year_in_school').choices
...
Solution 6 - Django
Something useful that worked for me when you have many complex methods calling each other and you need them available via your object:
First copy those model methods over into your migration file
def A(self):
return self.B() + self.C()
def B(self):
return self.name
def C(self):
return self.description
Then in your migration function:
def do_something_to_your_objects(apps, schema_editor):
MyModel = apps.get_model("my_app", "MyModel")
MyModel.A = A
MyModel.B = B
MyModel.C = C
for my_object in MyModel.objects.all():
my_object.name_and_decription = my_object.C()
my_object.save()
class Migration(migrations.Migration):
dependencies = [
('initial', '0001_initial'),
]
operations = [
migrations.RunPython(do_something_to_your_objects)
]