CODEWITHSUNDEEP
django
echoturing
echoturing

Reputation: 13

how django knows to update or insert

I'm reading django doc and see how django knows to do the update or insert method when calling save(). The doc says: 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 link.

But in practice, when I create a new instance of a Model and set its "id" property to a value that already exist in my database records. For example: I have a Model class named "User" and have a propery named "name".Just like below:

class User(model.Model):
    name=model.CharField(max_length=100)

Then I create a new User and save it:

user = User(name="xxx")
user.save()

now in my database table, a record like id=1, name="xxx" exists. Then I create a new User and just set the propery id=1:

newuser = User(id=1）
newuser.save()

not like the doc says.when I had this down.I checked out two records in my database table.One is id = 1 ,another is id=2

So, can anyone explain this to me? I'm confused.Thanks!

Upvotes: 1

Views: 4386

Answers (1)

brainless coder
brainless coder

Reputation: 6430

Because in newer version of django ( 1.5 > ), django does not check whether the id is in the database or not. So this could depend on the database. If the database report that this is duplicate, then it will update and if the database does not report it then it will insert. Check the doc -

In Django 1.5 and earlier, Django did a SELECT when the primary key attribute was set. If the SELECT found a row, then Django did an UPDATE, otherwise it did an INSERT. The old algorithm results in one more query in the UPDATE case. There are some rare cases where the database doesn’t report that a row was updated even if the database contains a row for the object’s primary key value. An example is the PostgreSQL ON UPDATE trigger which returns NULL. In such cases it is possible to revert to the old algorithm by setting the select_on_save option to True.

https://docs.djangoproject.com/en/1.8/ref/models/instances/#how-django-knows-to-update-vs-insert

But if you want this behavior, set select_on_save option to True.

You might wanna try force_update if that is required -

https://docs.djangoproject.com/en/1.8/ref/models/instances/#forcing-an-insert-or-update

Upvotes: 1

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