There are many Stack Overflow posts about recursion using the post_save signal, to which the comments and answers are overwhelmingly: "why not override save()" or a save that is only fired upon created == True.
Well I believe there's a good case for not using save() - for example, I am adding a temporary application that handles order fulfillment data completely separate from our Order model.
The rest of the framework is blissfully unaware of the fulfillment application and using post_save hooks isolates all fulfillment related code from our Order model.
If we drop the fulfillment service, nothing about our core code has to change. We delete the fulfillment app, and that's it.
So, are there any decent methods to ensure the post_save signal doesn't fire the same handler twice?
you can use update instead of save in the signal handler
queryset.filter(pk=instance.pk).update(....)
What you think about this solution?
#receiver(post_save, sender=Article)
def generate_thumbnails(sender, instance=None, created=False, **kwargs):
if not instance:
return
if hasattr(instance, '_dirty'):
return
do_something()
try:
instance._dirty = True
instance.save()
finally:
del instance._dirty
You can also create decorator
def prevent_recursion(func):
#wraps(func)
def no_recursion(sender, instance=None, **kwargs):
if not instance:
return
if hasattr(instance, '_dirty'):
return
func(sender, instance=instance, **kwargs)
try:
instance._dirty = True
instance.save()
finally:
del instance._dirty
return no_recursion
#receiver(post_save, sender=Article)
#prevent_recursion
def generate_thumbnails(sender, instance=None, created=False, **kwargs):
do_something()
Don't disconnect signals. If any new model of the same type is generated while the signal is disconnected the handler function won't be fired. Signals are global across Django and several requests can be running concurrently, making some fail while others run their post_save handler.
I think creating a save_without_signals() method on the model is more explicit:
class MyModel()
def __init__():
# Call super here.
self._disable_signals = False
def save_without_signals(self):
"""
This allows for updating the model from code running inside post_save()
signals without going into an infinite loop:
"""
self._disable_signals = True
self.save()
self._disable_signals = False
def my_model_post_save(sender, instance, *args, **kwargs):
if not instance._disable_signals:
# Execute the code here.
How about disconnecting then reconnecting the signal within your post_save function:
def my_post_save_handler(sender, instance, **kwargs):
post_save.disconnect(my_post_save_handler, sender=sender)
instance.do_stuff()
instance.save()
post_save.connect(my_post_save_handler, sender=sender)
post_save.connect(my_post_save_handler, sender=Order)
You should use queryset.update() instead of Model.save() but you need to take care of something else:
It's important to note that when you use it, if you want to use the new object you should get his object again, because it will not change the self object, for example:
>>> MyModel.objects.create(pk=1, text='')
>>> el = MyModel.objects.get(pk=1)
>>> queryset.filter(pk=1).update(text='Updated')
>>> print el.text
>>> ''
So, if you want to use the new object you should do again:
>>> MyModel.objects.create(pk=1, text='')
>>> el = MyModel.objects.get(pk=1)
>>> queryset.filter(pk=1).update(text='Updated')
>>> el = MyModel.objects.get(pk=1) # Do it again
>>> print el.text
>>> 'Updated'
You could also check the raw argument in post_save and then call save_baseinstead of save.
the Model's .objects.update() method bypasses the post_save signal
Try this something like this:
from django.db import models
from django.db.models.signals import post_save
class MyModel(models.Model):
name = models.CharField(max_length=200)
num_saves = models.PositiveSmallIntegerField(default=0)
#classmethod
def post_save(cls, sender, instance, created, *args, **kwargs):
MyModel.objects.filter(id=instance.id).update(save_counter=instance.save_counter + 1)
post_save.connect(MyModel.post_save, sender=MyModel)
In this example, an object has a name and each time .save() is called, the .num_saves property is incremented, but without recursion.
Check this out...
Each signal has it's own benefits as you can read about in the docs here but I wanted to share a couple things to keep in mind with the pre_save and post_save signals.
Both are called every time .save() on a model is called. In other words, if you save the model instance, the signals are sent.
running save() on the instance within a post_save can often create a never ending loop and therefore cause a max recursion depth exceeded error --- only if you don't use .save() correctly.
pre_save is great for changing just instance data because you do not have to call save() ever which eliminates the possibility for above. The reason you don't have to call save() is because a pre_save signal literally means right before being saved.
Signals can call other signals and or run delayed tasks (for Celery) which can be huge for usability.
Source: https://www.codingforentrepreneurs.com/blog/post-save-vs-pre-save-vs-override-save-method/
Regards!!
In post_save singal in django for avoiding recursion 'if created' check is required
from django.dispatch import receiver
from django.db.models.signals import post_save
#receiver(post_save, sender=DemoModel)
def _post_save_receiver(sender,instance,created, **kwargs):
if created:
print('hi..')
instance.save()
I was using the save_without_signals() method by #Rune Kaagaard until i updated my Django to 4.1. On Django 4.1 this method started raising an Integrity error on the database that gave me 4 days of headaches and i couldn't fix it.
So i started to use the queryset.update() method and it worked like a charm. It doesn't trigger the pre_save() neither post_save() and you don't need to override the save() method of your model. 1 line of code.
#receiver(pre_save, sender=Your_model)
def any_name(sender, instance, **kwargs):
Your_model.objects.filter(pk=instance.pk).update(model_attribute=any_value)
Related
How to do something after a Django user model save, including related changes to m2m fields like django.contrib.auth.models.Group?
Situation
I have a custom Django user model and want to trigger some actions after a user instance is - with related changes such as m2m group memberships - successfully saved to the database.
The use case here is a Wagtail CMS where I create ProfilePages for each user instance. Depending of the group memberships of the user instance, I need to do something.
Problem
In a custom model save() method, I'm not able to reference the changed group memberships, as m2m's are saved after the saving of the user instance. Even if running my custom function after the super().save() call, new group memberships are not yet available.
But I need to get the new group memberships in order to do something depending of the new groups for that user.
What I've tried
[✘] Custom model save()
# file: users/models.py
class CustomUser(AbstractUser):
super().save(*args, **kwargs)
do_something()
[✘] Signal post_save
As the above simple save() method did not do the trick, I tried the post_save signal of the user model:
# file users/signals.py
#receiver(post_save, sender=get_user_model())
def handle_profilepage(sender, instance, created, **kwargs):
action = 'created' if created else 'updated'
do_something()
... but even here I always get the "old" values from the group membership back.
[✔] Signal: m2m_changed
I learnt that there is a m2m_changed signal with which I could monitor changes of the Users.groups(.through) table.
My following implementation did what I need:
#receiver(m2m_changed, sender=User.groups.through)
def user_groups_changed_handler(sender, instance, **kwargs):
USER_GROUPS = instance.groups.values_list('name', flat=True)
if set(USER_GROUPS) & set(settings.PROFILE_GROUPS):
do_something_because_some_groups_match()
else:
do_something_else()
My whishlist
I would happily stay away from signals if there is a chance to solve this problem in the models save() method - but I'm stuck...
You asked after the dark magic. Tell me, what are you willing to sacrifice to avoid using signals? Would you embrace a greater evil? What if I were to suggest that you could start a new thread and sleep for a few seconds until you thought that the m2m relations were probably saved before doing anything?
from threading import Thread
from time import sleep
# file: users/models.py
class CustomUser(AbstractUser):
super().save(*args, **kwargs)
def do_something(obj):
sleep(3)
# stuff
thread = Thread(target = do_something, args = [self], daemon=True)
thread.start()
How to rewrite the Django model save method?
class Message(models.Model):
"""
message
"""
message_num = models.CharField(default=getMessageNum, max_length=16)
title = models.CharField(max_length=64)
content = models.CharField(max_length=1024)
def save(self, force_insert=False, force_update=False, using=None,
update_fields=None):
# I want send email there
pass
I mean, in the Django model, if I create instance success, I want to call a function, such as send a email in the function.
I find in the Django model have a save method. I am not sure whether should write other code, because there are so many params.
I mean whether I only should care about my send email logic?
When you override the save method, you still have to make sure that the it actually saves the instance. You can do that by simply calling the parent class' save via super:
class Message(models.Model):
# ...
def save(self, *args, **kwargs):
# this will take care of the saving
super(Message, self).save(*args, **kwargs)
# do email stuff
# better handle ecxeptions well or the saving might be rolled back
You can also connect the mail sending to the post_save (or pre_save, depending on your logic) signal. Whether you want to separate one orm the other in that way depends on how closely the two actions are linked and a bit on your taste.
Overriding save gives you the option to intervene in the saving process, e.g. you can change the value of fields based on whether the mail sending was successful or not save the instance at all.
The solution to what you want to do is to use Django Signals. By using Signals you can hook code to when a model is created and saved without having to rewrite the save method, that keep the separation of code and logic in a much nicer way, obviously the model does not need to know about the emails for example.
An example of how to use Signals would be to simply do the following:
from django.db.models.signals import pre_save
from django.dispatch import receiver
from myapp.models import MyModel
#receiver(pre_save, sender=MyModel)
def my_handler(sender, **kwargs):
# Code to execute whenever MyModel is saved...
If you still want to override the save() method you can use the Python super() method to do so (docs).
class MyModel(models.Model):
def save(self, *args, **kwargs):
# This will call the parent method that you are overriding
# so it will save your instance with the default behavior.
super(MyModel, self).save(*args, **kwargs)
# Then we add whatever extra code we want, e.g. send email...
Messenger.send_email()
You need to activate signal once your message is saved. That means, when your message is saved, django will issue signal as follows:
from django.db.models.signals import post_save
from django.dispatch import receiver
class Message(models.Model):
# fields...
# method for sending email
#receiver(post_save, sender=Message, dispatch_uid="send_email")
def send_email(sender, instance, **kwargs):
# your email send logic here..
You can put your signals in signals.py file inside your app folder and make sure to import that in your application config file as follows:
message/apps.py
from django.apps import AppConfig
class MyAppConfig(AppConfig):
name = 'message'
def ready(self):
import message.signals
And update init file as follows:
message/__init__.py
default_app_config = 'message.apps.MyAppConfig'
I need to do some actions when one field has changed.
Since this action needs to work with already saved object, I can't use pre_save signal like this:
#receiver(pre_save, sender=reservation_models.Reservation)
def generate_possible_pairs(sender, instance, **kwargs):
try:
reservation_old = sender.objects.get(pk=instance.pk)
except sender.DoesNotExist:
pass # Object is new, so field hasn't technically changed, but you may want to do something else here.
else:
if not reservation_old.datetime == instance.datetime: # Field has changed
do_something(instance) # It would be better to be sure instance has been saved
Is it possible to use post_save signal for this?
I would like to avoid adding temporary attributes to this model.
Using the post_save signal you won't be able to retrieve the previous state from db - But why use a signal at all ?
class Reservation(models.Model):
def save(self, *args, **kw):
old = type(self).objects.get(pk=self.pk) if self.pk else None
super(Reservation, self).save(*args, **kw)
if old and old.datetime != self.datetime: # Field has changed
do_something(self)
You may also want to read this : https://lincolnloop.com/blog/django-anti-patterns-signals/
Yes you can use a post_save too. You should however remember signals are synchronous
I read the django docs about signals and wrote this piece of code for my model Car :
#receiver(request_finished)
def signal_callback(sender, **kwargs):
print 'Save Signal received'
#receiver(post_save, sender=Car)
def signal_handler(sender, **kwargs):
pass
request_finished(signal_callback, sender=car, dispatch_url="Unique save id")
But the problem is, that when I fire up my server, and just open up the admin, I get a lot of 'Save Signal received' in my terminal. What I am wondering about is I have restricted the signal_handler to post_save only. But still, without even saving anything, the message shows up a lot of times. I dont understand this.
Note : I will be honest. I understood parts of it, not everything from the documentation.
There is a simpler way to bind post_save signals
from django.db.models.signals import post_save
from myapp.models import Car
def do_something(sender, **kwargs):
print 'the object is now saved.'
car = kwargs['instance'] #now i have access to the object
post_save.connect(do_something, sender=Car)
The signal request finished gets called every time a HTTP request is made, which is a hog.
You binded request_finished signal to signal_callback. Remove(or comment out) signal_callback, and change signal_handler as follow.
#receiver(post_save, sender=Car)
def signal_handler(sender, **kwargs):
print 'Save signal received'
I have a few model inheritance levels in Django:
class WorkAttachment(models.Model):
""" Abstract class that holds all fields that are required in each attachment """
work = models.ForeignKey(Work)
added = models.DateTimeField(default=datetime.datetime.now)
views = models.IntegerField(default=0)
class Meta:
abstract = True
class WorkAttachmentFileBased(WorkAttachment):
""" Another base class, but for file based attachments """
description = models.CharField(max_length=500, blank=True)
size = models.IntegerField(verbose_name=_('size in bytes'))
class Meta:
abstract = True
class WorkAttachmentPicture(WorkAttachmentFileBased):
""" Picture attached to work """
image = models.ImageField(upload_to='works/images', width_field='width', height_field='height')
width = models.IntegerField()
height = models.IntegerField()
There are many different models inherited from WorkAttachmentFileBased and WorkAttachment. I want to create a signal, which would update an attachment_count field for parent work, when attachment is created. It would be logical, to think that signal made for parent sender (WorkAttachment) would run for all inherited models too, but it does not. Here is my code:
#receiver(post_save, sender=WorkAttachment, dispatch_uid="att_post_save")
def update_attachment_count_on_save(sender, instance, **kwargs):
""" Update file count for work when attachment was saved."""
instance.work.attachment_count += 1
instance.work.save()
Is there a way to make this signal work for all models inherited from WorkAttachment?
Python 2.7, Django 1.4 pre-alpha
P.S. I've tried one of the solutions I found on the net, but it did not work for me.
You could register the connection handler without sender specified. And filter the needed models inside it.
from django.db.models.signals import post_save
from django.dispatch import receiver
#receiver(post_save)
def my_handler(sender, **kwargs):
# Returns false if 'sender' is NOT a subclass of AbstractModel
if not issubclass(sender, AbstractModel):
return
...
Ref: https://groups.google.com/d/msg/django-users/E_u9pHIkiI0/YgzA1p8XaSMJ
The simplest solution is to not restrict on the sender, but to check in the signal handler whether the respective instance is a subclass:
#receiver(post_save)
def update_attachment_count_on_save(sender, instance, **kwargs):
if isinstance(instance, WorkAttachment):
...
However, this may incur a significant performance overhead as every time any model is saved, the above function is called.
I think I've found the most Django-way of doing this: Recent versions of Django suggest to connect signal handlers in a file called signals.py. Here's the necessary wiring code:
your_app/__init__.py:
default_app_config = 'your_app.apps.YourAppConfig'
your_app/apps.py:
import django.apps
class YourAppConfig(django.apps.AppConfig):
name = 'your_app'
def ready(self):
import your_app.signals
your_app/signals.py:
def get_subclasses(cls):
result = [cls]
classes_to_inspect = [cls]
while classes_to_inspect:
class_to_inspect = classes_to_inspect.pop()
for subclass in class_to_inspect.__subclasses__():
if subclass not in result:
result.append(subclass)
classes_to_inspect.append(subclass)
return result
def update_attachment_count_on_save(sender, instance, **kwargs):
instance.work.attachment_count += 1
instance.work.save()
for subclass in get_subclasses(WorkAttachment):
post_save.connect(update_attachment_count_on_save, subclass)
I think this works for all subclasses, because they will all be loaded by the time YourAppConfig.ready is called (and thus signals is imported).
You could try something like:
model_classes = [WorkAttachment, WorkAttachmentFileBased, WorkAttachmentPicture, ...]
def update_attachment_count_on_save(sender, instance, **kwargs):
instance.work.attachment_count += 1
instance.work.save()
for model_class in model_classes:
post_save.connect(update_attachment_count_on_save,
sender=model_class,
dispatch_uid="att_post_save_"+model_class.__name__)
(Disclaimer: I have not tested the above)
I just did this using python's (relatively) new __init_subclass__ method:
from django.db import models
def perform_on_save(*args, **kw):
print("Doing something important after saving.")
class ParentClass(models.Model):
class Meta:
abstract = True
#classmethod
def __init_subclass__(cls, **kwargs):
super().__init_subclass__(**kwargs)
models.signals.post_save.connect(perform_on_save, sender=cls)
class MySubclass(ParentClass):
pass # signal automatically gets connected.
This requires django 2.1 and python 3.6 or better. Note that the #classmethod line seems to be required when working with the django model and associated metaclass even though it's not required according to the official python docs.
post_save.connect(my_handler, ParentClass)
# connect all subclasses of base content item too
for subclass in ParentClass.__subclasses__():
post_save.connect(my_handler, subclass)
have a nice day!
Michael Herrmann's solution is definitively the most Django-way of doing this.
And yes it works for all subclasses as they are loaded at the ready() call.
I would like to contribute with the documentation references :
In practice, signal handlers are usually defined in a signals submodule of the application they relate to. Signal receivers are connected in the ready() method of your application configuration class. If you’re using the receiver() decorator, simply import the signals submodule inside ready().
https://docs.djangoproject.com/en/dev/topics/signals/#connecting-receiver-functions
And add a warning :
The ready() method may be executed more than once during testing, so you may want to guard your signals from duplication, especially if you’re planning to send them within tests.
https://docs.djangoproject.com/en/dev/topics/signals/#connecting-receiver-functions
So you might want to prevent duplicate signals with a dispatch_uid parameter on the connect function.
post_save.connect(my_callback, dispatch_uid="my_unique_identifier")
In this context I'll do :
for subclass in get_subclasses(WorkAttachment):
post_save.connect(update_attachment_count_on_save, subclass, dispatch_uid=subclass.__name__)
https://docs.djangoproject.com/en/dev/topics/signals/#preventing-duplicate-signals
This solution resolves the problem when not all modules imported into memory.
def inherited_receiver(signal, sender, **kwargs):
"""
Decorator connect receivers and all receiver's subclasses to signals.
#inherited_receiver(post_save, sender=MyModel)
def signal_receiver(sender, **kwargs):
...
"""
parent_cls = sender
def wrapper(func):
def childs_receiver(sender, **kw):
"""
the receiver detect that func will execute for child
(and same parent) classes only.
"""
child_cls = sender
if issubclass(child_cls, parent_cls):
func(sender=child_cls, **kw)
signal.connect(childs_receiver, **kwargs)
return childs_receiver
return wrapper
It's also possible to use content types to discover subclasses - assuming you have the base class and subclasses packaged in the same app. Something like this would work:
from django.contrib.contenttypes.models import ContentType
content_types = ContentType.objects.filter(app_label="your_app")
for content_type in content_types:
model = content_type.model_class()
post_save.connect(update_attachment_count_on_save, sender=model)
In addition to #clwainwright answer, I configured his answer to instead work for the m2m_changed signal. I had to post it as an answer for the code formatting to make sense:
#classmethod
def __init_subclass__(cls, **kwargs):
super().__init_subclass__(**kwargs)
for m2m_field in cls._meta.many_to_many:
if hasattr(cls, m2m_field.attname) and hasattr(getattr(cls, m2m_field.attname), 'through'):
models.signals.m2m_changed.connect(m2m_changed_receiver, weak=False, sender=getattr(cls, m2m_field.attname).through)
It does a couple of checks to ensure it doesn't break if anything changes in future Django versions.