CODEWITHSUNDEEP
pythondjango
David542
David542

Reputation: 110203

UPDATE doesnt use Model save method

If I do the folllowing:

obj = Model.objects.get(pk=2)
object.field = 'new value'
object.save()

It runs the custom save method that I have written in django.

However, if I do a normal update statement:

Model.objects.filter(pk=2).update(field='new value')

It does not use the custom save method. My question here is two-fold:

Upvotes: 1

Views: 96

Answers (1)

Cody Piersall
Cody Piersall

Reputation: 8547

I'm not a Django developer, but I dabble from time to time and no one else has answered yet.

Why was that decision made in django -- why doesn't every 'save' implement the custom save method.

I'm going to guess here that this is done as a speed optimization for the common case of just performing a bulk update. update works on the SQL level so it is potentially much faster than calling save on lots of objects, each one being its own database transaction.

Is there a codebase-wide way to make sure that no update statements are ever made? Basically, I just want to ensure that the custom save method is always run whenever doing a save within the django ORM. How would this be possible?

You can use a custom manager with a custom QuerySet that raises some Exception whenever update is called. Of course, you can always loop over the Queryset and call save on each object if you need the custom behavior.

Forbidding Update on a Model

from django.db import models

class NoUpdateQuerySet(models.QuerySet):
    """Don't let people call update! Muahaha"""
    def update(self, **kwargs):
        # you should raise a more specific Exception.
        raise Exception('You shall not update; use save instead.')

class Person(models.Model):
    first_name = models.CharField(max_length=50)
    last_name = models.CharField(max_length=50)

    # setting the custom manager keeps people from calling update.
    objects = NoUpdateQuerySet.as_manager()

You would just need to set the NoUpdateQuerySet as a manager for each model you don't want to update. I don't really think it's necessary to set a custom QuerySet though; if it were me I would just not call update, and loop through the objects that need to be saved whenever I need to. You may find a time when you want to call update, and this would end up being very annoying.

Forbidding Update Project-Wide

If you really really decide you hate update, you can just monkey-patch the update method. Then you can be completely certain it's not being called. You can monkey-patch it in your project's settings.py, since you know that module will be imported:

def no_update(self, **kwargs):
    # probably want a more specific Exception
    raise Exception('NO UPDATING HERE')

from django.db.models.query import QuerySet
QuerySet.update = no_update

Note that the traceback will actually be pretty confusing, since it will point to a function in settings.py. I'm not sure how much, if ever, update is used by other apps; this could have unintended consequences.

Upvotes: 2

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