Django Model() vs Model.objects.create()

Question

What it the difference between running two commands:

foo = FooModel()

and

bar = BarModel.objects.create()

Answer 1

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")

Answer 2

The difference between Model() vs Model.objects.create() are summarized as below.

---

1. .save() perform internally as either INSERT or UPDATE object to db, while .objects.create() perform only INSERT object to db.

   Model.save() perform ....

   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).

---

2. If primary key attribute is set to a value then Model.save() perform UPDATE but Model.objects.create raise IntegrityError.

   eg.

   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()
   

   1) 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()
   

   Output:

   Subject.objects.all().values()
   <QuerySet [{'subject_id': 1, 'name': 'Math', 'max_marks': 50}]>
   

   2) Insert to db with Model.objects.create()

   Subject.objects.create(subject_id=1, name='Chemistry', max_marks=100)
   IntegrityError: UNIQUE constraint failed: m****t.subject_id
   

   Explanation: Above math.save() is case of update since subject_id is primary key and subject_id=1 exists django internally perform UPDATE, name Physics to Math and max_marks from 100 to 50 for this, but objects.create() raise IntegrityError

---

3. Model.objects.create() not equivalent to Model.save() however same can be achieved with force_insert=True parameter on save method i.e Model.save(force_insert=True).

---

4. Model.save() return None where Model.objects.create() return model instance i.e. package_name.models.Model

---

Conclusion: Model.objects.create() internally do model initialization and perform save with force_insert=True.

source-code block 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

The following links can be followed for more details:

1. https://docs.djangoproject.com/en/stable/ref/models/querysets/#create

2. https://github.com/django/django/blob/2d8dcba03aae200aaa103ec1e69f0a0038ec2f85/django/db/models/query.py#L440

Answer 3

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.

Answer 4

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