What it the difference between running two commands:
foo = FooModel()
and
bar = BarModel.objects.create()
Does the second one immediately create a BarModel
in the database, while for FooModel
, the save()
method has to be called explicitly to add it to the database?
https://docs.djangoproject.com/en/stable/topics/db/queries/#creating-objects
To create and save an object in a single step, use the create() method.
The differences between Model()
and Model.objects.create()
are the following:
INSERT vs UPDATE Model.save() does either INSERT or UPDATE of an object in a DB, while Model.objects.create() does only INSERT. Model.save() does UPDATE If the object’s primary key attribute is set to a value that evaluates to True INSERT If the object’s primary key attribute is not set or if the UPDATE didn’t update anything (e.g. if primary key is set to a value that doesn’t exist in the database).
Existing primary key If primary key attribute is set to a value and such primary key already exists, then Model.save() performs UPDATE, but Model.objects.create() raises IntegrityError. Consider the following models.py: class Subject(models.Model):
subject_id = models.PositiveIntegerField(primary_key=True, db_column='subject_id')
name = models.CharField(max_length=255)
max_marks = models.PositiveIntegerField()
Insert/Update to db with Model.save() physics = Subject(subject_id=1, name='Physics', max_marks=100)
physics.save()
math = Subject(subject_id=1, name='Math', max_marks=50) # Case of update
math.save()
Result: Subject.objects.all().values()
Forced insert Model.save() can be made to behave as Model.objects.create() by using force_insert=True parameter: Model.save(force_insert=True).
Return value Model.save() return None where Model.objects.create() return model instance i.e. package_name.models.Model
Conclusion: Model.objects.create()
does model initialization and performs save()
with force_insert=True
.
Excerpt from the source code of Model.objects.create()
def create(self, **kwargs):
"""
Create a new object with the given kwargs, saving it to the database
and returning the created object.
"""
obj = self.model(**kwargs)
self._for_write = True
obj.save(force_insert=True, using=self.db)
return obj
For more details follow the links:
https://docs.djangoproject.com/en/stable/ref/models/querysets/#create https://github.com/django/django/blob/2d8dcba03aae200aaa103ec1e69f0a0038ec2f85/django/db/models/query.py#L440
The two syntaxes are not equivalent and it can lead to unexpected errors. Here is a simple example showing the differences. If you have a model:
from django.db import models
class Test(models.Model):
added = models.DateTimeField(auto_now_add=True)
And you create a first object:
foo = Test.objects.create(pk=1)
Then you try to create an object with the same primary key:
foo_duplicate = Test.objects.create(pk=1)
# returns the error:
# django.db.utils.IntegrityError: (1062, "Duplicate entry '1' for key 'PRIMARY'")
foo_duplicate = Test(pk=1).save()
# returns the error:
# django.db.utils.IntegrityError: (1048, "Column 'added' cannot be null")
.create()
creates an object even if an required field(null=False
) is missing? I am adding tests to my project and create
is having unexpected results
CharField
even if set to null=False
will not raise an error if not provided: this is because Django set strings by default to an empty string ""
so it is not technically null
obj = MyModel()
, then obj.full_clean()
for now.
UPDATE 15.3.2017:
I have opened a Django-issue on this and it seems to be preliminary accepted here: https://code.djangoproject.com/ticket/27825
My experience is that when using the Constructor
(ORM
) class by references with Django 1.10.5
there might be some inconsistencies in the data (i.e. the attributes of the created object may get the type of the input data instead of the casted type of the ORM object property) example:
models
class Payment(models.Model):
amount_cash = models.DecimalField()
some_test.py
- object.create
Class SomeTestCase:
def generate_orm_obj(self, _constructor, base_data=None, modifiers=None):
objs = []
if not base_data:
base_data = {'amount_case': 123.00}
for modifier in modifiers:
actual_data = deepcopy(base_data)
actual_data.update(modifier)
# Hacky fix,
_obj = _constructor.objects.create(**actual_data)
print(type(_obj.amount_cash)) # Decimal
assert created
objs.append(_obj)
return objs
some_test.py
- Constructor()
Class SomeTestCase:
def generate_orm_obj(self, _constructor, base_data=None, modifiers=None):
objs = []
if not base_data:
base_data = {'amount_case': 123.00}
for modifier in modifiers:
actual_data = deepcopy(base_data)
actual_data.update(modifier)
# Hacky fix,
_obj = _constructor(**actual_data)
print(type(_obj.amount_cash)) # Float
assert created
objs.append(_obj)
return objs
Model.objects.create()
creates a model instance and saves it. Model()
only creates an in memory model instance. It's not saved to the database until you call the instance's save()
method to save it. That's when validation happens also.
Success story sharing