CODEWITHSUNDEEP
djangopostgresqlpostgresql-9.5django-1.9
Sidharth Samant
Sidharth Samant

Reputation: 786

Django default at database

So as I recently came to understand, the default that we specify in a Django field is merely for filling up pre-existing rows, and doesn't function as a "real" default value at the database level. This can be verified by looking up column_default from the database shell.

If I understand correctly, default is used by Postgres to put in values when there is no input. If that is so, then we shouldn't be able to create a new object in Django without providing the default value, right?

For ex - 

class MyModel(models.Model):
    name = models.CharField(max_length=255)
    status = models.BooleanField(default=True) # new field added

For the above model, the default is used for only backfilling the newly added field for pre-existing rows. It doesn't actually set a default at the database level.

 column_name | column_default                                     
-------------+----------------
 mymodel     |

But if that is so, I shouldn't be able to run this query - 

MyModel.objects.create(name='test')

since there's no "real" default. But I can, and I don't understand why.

Thank you!

Upvotes: 5

Views: 2921

Answers (1)

dani herrera
dani herrera

Reputation: 51705

Quoting docs:

The default value is used when new model instances are created and a value isn’t provided for the field. When the field is a primary key, the default is also used when the field is set to None.

Default works at application level, not at database level. I mean, when new model instance is created (in memory) default value is applied there is not an explicit value provided.

Default is not translated to ddl ( database data definition language ) as not null or unique do.

Notice that default may be a value or, alseo, may be a django function that is invoked when new model is created to provide a value.

Because all this, the default field value will not be present on database schema.

Edited 2023

Use the new db_default for database-computed default value (credits to @sytech, see comments)

Upvotes: 7

Related Questions