#30844: Add after_db_init() hook method to model
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Reporter: rsinger86 | Owner: nobody
Type: Uncategorized | Status: new
Component: Uncategorized | Version: 2.2
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 1
UI/UX: 0 |
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I've encountered a need in my personal projects, and when writing
[https://github.com/rsinger86/django-lifecycle django-lifecycle], to hook
into the moment when a model has been fully initialized from the database.
Overriding the model's __init__ method does NOT work here because the
`select_related` relationships have not yet been added in/cached in the
model's FK fields. It would be useful to do things right after the model
is fully loaded and initialized from the database. For example, if you
have a "type" foreign key field, you may want to apply some polymorphic
behavior based on the value of that field. Doing this in `__init__` will
cause a n+1 query explosion if you do it when iterating over a `QuerySet`.
Causes n+1 issue when iterating a QuerySet:
{{{
class CatMixin(object):
def greet(self):
return "Meow"
class DogMixin(object):
def greet(self):
return "Woof"
class PolymorphicModel(models.Model):
type = models.ForeignKey(PetType)
def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)
if type.name == 'cat':
self.__class__ = CatMixin
else:
self.__class__ = DogMixin
}}}
The fix: Add a call to `obj.post_db_init()` to this line:
https://github.com/django/django/blob/master/django/db/models/query.py#L91
Then this code will eliminate the n+1 problem (assuming
`select_related('type')` is used):
{{{
class PolymorphicModel(models.Model):
type = models.ForeignKey(PetType)
def post_db_init(self,):
if type.name == 'cat':
self.__class__ = CatMixin
else:
self.__class__ = DogMixin
}}}
I realize there are other ways to achieve the behavior in this example --
I'm bringing up polymorphism as a general use case. In django-lifecycle,
this hook would allow me to track a model instance's initial state across
a foreign key relationship without causing users to experience the n+1
problem.
--
Ticket URL: <https://code.djangoproject.com/ticket/30844>
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