Constrains

This is an additional option within the Meta class, utilized for establishing certain constraints on the model. Implementing the unique=True option on a field effectively acts as a constraint.

UniqueConstraint

Ensures that the combination of specified field values is unique across the database table.

python
99
1
2
3
4
5
6
7
8
9
10
11
12
from django.db import models
from django.db.models import UniqueConstraint

class Book(models.Model):
    title = models.CharField(max_length=100)
    genre = models.ForeignKey(Genre, on_delete=models.CASCADE)
    author = models.ForeignKey(Author, on_delete=models.CASCADE)

    class Meta:
        constraints = [
            UniqueConstraint(fields=["title", "genre"], name="unique_book_titles_per_genre")
        ]

In this example, the UniqueConstraint ensures that no two books in the same genre have the same title.

CheckConstraint

Ensures that certain conditions hold true for the data in the database.

Suppose you want to ensure that the author's age is always greater than 18.

python
99
1
2
3
4
5
6
7
8
9
10
from django.db.models import CheckConstraint, Q

class Author(models.Model):
    name = models.CharField(max_length=100)
    age = models.IntegerField()

    class Meta:
        constraints = [
            CheckConstraint(check=Q(age__gt=18), name="age_greater_than_18")
        ]