How to update() a single model instance retrieved by get() on Django ORM?
Question:
I have a function which currently calls Models.object.get()
, which returns either 0 or 1 model objects:
- if it returns 0, I create a new model instance in the
except DoesNotExist
clause of the function.
- Otherwise, I would like to update the fields in the pre-existing
instance, without creating a new one.
I was originally attempting to
call .update()
on the instance which was found, but .update()
seems to be only callable on a QuerySets. How do I get around
changing a dozen fields, without calling .filter()
and comparing
the lengths to know if I have to create or update a pre-existing
instance?
Answers:
With the advent of Django 1.7, there is now a new update_or_create
QuerySet method, which should do exactly what you want. Just be careful of potential race conditions if uniqueness is not enforced at the database level.
Example from the documentation:
obj, created = Person.objects.update_or_create(
first_name='John', last_name='Lennon',
defaults={'first_name': 'Bob'},
)
The update_or_create
method tries to fetch an object from database
based on the given kwargs. If a match is found, it updates the
fields passed in the defaults
dictionary.
Pre-Django 1.7:
Change the model field values as appropriate, then call .save()
to persist the changes:
try:
obj = Model.objects.get(field=value)
obj.field = new_value
obj.save()
except Model.DoesNotExist:
obj = Model.objects.create(field=new_value)
# do something else with obj if need be
I don’t know how good or bad this is, but you can try something like this:
try:
obj = Model.objects.get(id=some_id)
except Model.DoesNotExist:
obj = Model.objects.create()
obj.__dict__.update(your_fields_dict)
obj.save()
I am using the following code in such cases:
obj, created = Model.objects.get_or_create(id=some_id)
if not created:
resp= "It was created"
else:
resp= "OK"
obj.save()
As of Django 1.5, there is an update_fields property on model save. eg:
obj.save(update_fields=['field1', 'field2', ...])
https://docs.djangoproject.com/en/dev/ref/models/instances/
I prefer this approach because it doesn’t create an atomicity problem if you have multiple web app instances changing different parts of a model instance.
if you want only to update model if exist (without create it):
Model.objects.filter(id = 223).update(field1 = 2)
mysql query:
UPDATE `model` SET `field1` = 2 WHERE `model`.`id` = 223
Here’s a mixin that you can mix into any model class which gives each instance an update
method:
class UpdateMixin(object):
def update(self, **kwargs):
if self._state.adding:
raise self.DoesNotExist
for field, value in kwargs.items():
setattr(self, field, value)
self.save(update_fields=kwargs.keys())
The self._state.adding
check checks to see if the model is saved to the database, and if not, raises an error.
(Note: This update
method is for when you want to update a model and you know the instance is already saved to the database, directly answering the original question. The built-in update_or_create
method featured in Platinum Azure’s answer already covers the other use-case.)
You would use it like this (after mixing this into your user model):
user = request.user
user.update(favorite_food="ramen")
Besides having a nicer API, another advantage to this approach is that it calls the pre_save
and post_save
hooks, while still avoiding atomicity issues if another process is updating the same model.
As @Nils mentionned, you can use the update_fields
keyword argument of the save()
method to manually specify the fields to update.
obj_instance = Model.objects.get(field=value)
obj_instance.field = new_value
obj_instance.field2 = new_value2
obj_instance.save(update_fields=['field', 'field2'])
The update_fields
value should be a list of the fields to update as strings.
See https://docs.djangoproject.com/en/2.1/ref/models/instances/#specifying-which-fields-to-save
update:
1 – individual instance :
get instance and update manually get() retrieve individual object
post = Post.objects.get(id=1)
post.title = "update title"
post.save()
2 – Set of instances :
use update() method that works only with queryset that what would be returned by filter() method
Post.objects.filter(author='ahmed').update(title='updated title for ahmed')
I have a function which currently calls Models.object.get()
, which returns either 0 or 1 model objects:
- if it returns 0, I create a new model instance in the
except DoesNotExist
clause of the function. - Otherwise, I would like to update the fields in the pre-existing
instance, without creating a new one.
I was originally attempting to
call .update()
on the instance which was found, but .update()
seems to be only callable on a QuerySets. How do I get around
changing a dozen fields, without calling .filter()
and comparing
the lengths to know if I have to create or update a pre-existing
instance?
With the advent of Django 1.7, there is now a new update_or_create
QuerySet method, which should do exactly what you want. Just be careful of potential race conditions if uniqueness is not enforced at the database level.
Example from the documentation:
obj, created = Person.objects.update_or_create(
first_name='John', last_name='Lennon',
defaults={'first_name': 'Bob'},
)
The
update_or_create
method tries to fetch an object from database
based on the given kwargs. If a match is found, it updates the
fields passed in thedefaults
dictionary.
Pre-Django 1.7:
Change the model field values as appropriate, then call .save()
to persist the changes:
try:
obj = Model.objects.get(field=value)
obj.field = new_value
obj.save()
except Model.DoesNotExist:
obj = Model.objects.create(field=new_value)
# do something else with obj if need be
I don’t know how good or bad this is, but you can try something like this:
try:
obj = Model.objects.get(id=some_id)
except Model.DoesNotExist:
obj = Model.objects.create()
obj.__dict__.update(your_fields_dict)
obj.save()
I am using the following code in such cases:
obj, created = Model.objects.get_or_create(id=some_id)
if not created:
resp= "It was created"
else:
resp= "OK"
obj.save()
As of Django 1.5, there is an update_fields property on model save. eg:
obj.save(update_fields=['field1', 'field2', ...])
https://docs.djangoproject.com/en/dev/ref/models/instances/
I prefer this approach because it doesn’t create an atomicity problem if you have multiple web app instances changing different parts of a model instance.
if you want only to update model if exist (without create it):
Model.objects.filter(id = 223).update(field1 = 2)
mysql query:
UPDATE `model` SET `field1` = 2 WHERE `model`.`id` = 223
Here’s a mixin that you can mix into any model class which gives each instance an update
method:
class UpdateMixin(object):
def update(self, **kwargs):
if self._state.adding:
raise self.DoesNotExist
for field, value in kwargs.items():
setattr(self, field, value)
self.save(update_fields=kwargs.keys())
The self._state.adding
check checks to see if the model is saved to the database, and if not, raises an error.
(Note: This update
method is for when you want to update a model and you know the instance is already saved to the database, directly answering the original question. The built-in update_or_create
method featured in Platinum Azure’s answer already covers the other use-case.)
You would use it like this (after mixing this into your user model):
user = request.user
user.update(favorite_food="ramen")
Besides having a nicer API, another advantage to this approach is that it calls the pre_save
and post_save
hooks, while still avoiding atomicity issues if another process is updating the same model.
As @Nils mentionned, you can use the update_fields
keyword argument of the save()
method to manually specify the fields to update.
obj_instance = Model.objects.get(field=value)
obj_instance.field = new_value
obj_instance.field2 = new_value2
obj_instance.save(update_fields=['field', 'field2'])
The update_fields
value should be a list of the fields to update as strings.
See https://docs.djangoproject.com/en/2.1/ref/models/instances/#specifying-which-fields-to-save
update:
1 – individual instance :
get instance and update manually get() retrieve individual object
post = Post.objects.get(id=1)
post.title = "update title"
post.save()
2 – Set of instances :
use update() method that works only with queryset that what would be returned by filter() method
Post.objects.filter(author='ahmed').update(title='updated title for ahmed')