I want to create a model object, like Person, if person’s id doesn’t not exist, or I will get that person object.

The code to create a new person as following:

class Person(models.Model):
    identifier = models.CharField(max_length = 10)
    name = models.CharField(max_length = 20)
    objects = PersonManager()

class PersonManager(models.Manager):
    def create_person(self, identifier):
        person = self.create(identifier = identifier)
        return person

But I don’t know where to check and get the existing person object.

If you’re looking for “update if exists else create” use case, please refer to @Zags excellent answer

Django already has a get_or_create, https://docs.djangoproject.com/en/dev/ref/models/querysets/#get-or-create

For you it could be :

id = 'some identifier'
person, created = Person.objects.get_or_create(identifier=id)

if created:
   # means you have created a new person
else:
   # person just refers to the existing one

It’s unclear whether your question is asking for the get_or_create method (available from at least Django 1.3) or the update_or_create method (new in Django 1.7). It depends on how you want to update the user object.

Sample use is as follows:

# In both cases, the call will get a person object with matching
# identifier or create one if none exists; if a person is created,
# it will be created with name equal to the value in `name`.

# In this case, if the Person already exists, its existing name is preserved
person, created = Person.objects.get_or_create(
        identifier=identifier, defaults={"name": name}
)

# In this case, if the Person already exists, its name is updated
person, created = Person.objects.update_or_create(
        identifier=identifier, defaults={"name": name}
)

Django has support for this, check get_or_create

person, created = Person.objects.get_or_create(name='abc')
if created:
    # A new person object created
else:
    # person object already exists

For only a small amount of objects the update_or_create works well, but if you’re doing over a large collection it won’t scale well. update_or_create always first runs a SELECT and thereafter an UPDATE.

for the_bar in bars:
    updated_rows = SomeModel.objects.filter(bar=the_bar).update(foo=100)
        if not updated_rows:
            # if not exists, create new
            SomeModel.objects.create(bar=the_bar, foo=100)

This will at best only run the first update-query, and only if it matched zero rows run another INSERT-query. Which will greatly increase your performance if you expect most of the rows to actually be existing.

It all comes down to your use case though. If you are expecting mostly inserts then perhaps the bulk_create() command could be an option.

Thought I’d add an answer since your question title looks like it is asking how to create or update, rather than get or create as described in the question body.

If you did want to create or update an object, the .save() method already has this behaviour by default, from the docs:

Django abstracts the need to use INSERT or UPDATE SQL statements. Specifically, when you call save(), Django follows this algorithm:

If the object’s primary key attribute is set to a value that evaluates to True (i.e., a value other than None or the empty string), Django executes an UPDATE. If the object’s primary key attribute is not set or if the UPDATE didn’t update anything, Django executes an INSERT.

It’s worth noting that when they say ‘if the UPDATE didn’t update anything’ they are essentially referring to the case where the id you gave the object doesn’t already exist in the database.

If one of the input when you create is a primary key, this will be enough:

Person.objects.get_or_create(id=1)

It will automatically update if exist since two data with the same primary key is not allowed.