Defining Model Fields

Declaring a Field

A field is declared as a class attribute set to a field class instance, such as models.CharField(...). Django uses this to build the matching database column.

Example: Declaring a Field

A field is declared as a class attribute set to a field class instance, such as models.CharField(...). Django uses this to build the matching database column.

from django.db import models


class Product(models.Model):
    name = models.CharField(max_length=100)
    price = models.IntegerField()

Required Field Arguments

Some fields require arguments to work correctly. CharField always needs max_length, since Django needs to know the column's maximum size at the database level.

Note: max_length is about database storage size, not form validation length.

Warning: Omitting max_length on a CharField raises a system check error before you can even migrate.

Example: Required Field Arguments

Some fields require arguments to work correctly. CharField always needs max_length, since Django needs to know the column's maximum size at the database level.

from django.db import models


class Comment(models.Model):
    author_name = models.CharField(max_length=80)
    body = models.TextField()

Optional Field Behavior

blank=True controls whether a field can be left empty in forms, while null=True controls whether the database column itself can store NULL. They solve different problems.

Note: For text-based fields, prefer blank=True with an empty string default rather than null=True, to avoid two ways of representing 'no value'.

Example: Optional Field Behavior

blank=True controls whether a field can be left empty in forms, while null=True controls whether the database column itself can store NULL. They solve different problems.

from django.db import models


class Profile(models.Model):
    bio = models.TextField(blank=True, default='')
    nickname = models.CharField(max_length=50, blank=True, null=True)

Default Values

The default argument gives a field a starting value whenever a new object is created without explicitly setting that field.

Example: Default Values

The default argument gives a field a starting value whenever a new object is created without explicitly setting that field.

from django.db import models


class Order(models.Model):
    quantity = models.IntegerField(default=1)
    is_paid = models.BooleanField(default=False)