How to add these constraints to Django models? - django

Question

Help me please. How to make constraints in the database?
I am using sqlite, django 3.2, python 3.7
I have three models: Refbook, Versions of refbooks, Elements of refbook
from django.db import models
class Refbook(models.Model):
code = models.CharField(max_length=100, unique=True)
name = models.CharField(max_length=300)
description = models.TextField()
class VersionRefbook(models.Model):
refbook_id = models.ForeignKey('Refbook',
on_delete=models.CASCADE,
related_name='versions')
version = models.CharField(max_length=50)
date = models.DateField()
class Meta:
unique_together = ('refbook_id', 'version')
class Element(models.Model):
version_id = models.ManyToManyField('VersionRefbook')
code = models.CharField(max_length=100)
value = models.CharField(max_length=300)
The following constraints must be added to these models:
There cannot be more than one Refbook with the same value in the "code" field.
There cannot be more than one Version with the same set of values "refbook id" and "version"
One Refbook cannot have more than one Version with the same date
In one Version of refbook, there cannot be more than one Element of refbook with the same value in the "code" field
OK. I solved the first point by adding the «unique=true» parameter to the «code» field in the Refbook class.
It seems that I solved the seconf point by adding the «unique_together = ('refbook_id', 'version')» parameter to the Meta class.
Third point. I think need to override the save method. But how? And is it right?
Fourth point. I wanted to add a parameter « unique_together = ('version_id', 'code')» to the Meta class of the Element class, but there is a connection ManyToMany there. It gives an error message.
Have I added the first and second constraints correctly? And how to solve the third and fourth?

Answer 1

Seems you have added the first two constraints correctly.
3. One Refbook cannot have more than one Version with the same date
Just add another unique constraint for version and date.
class VersionRefbook(models.Model):
refbook_id = models.ForeignKey(
'Refbook',
on_delete=models.CASCADE,
related_name='versions'
)
version = models.CharField(max_length=50)
date = models.DateField()
class Meta:
unique_together = ('refbook_id', 'version')
unique_together = ('version', 'date')
4. In one Version of refbook, there cannot be more than one Element of refbook with the same value in the "code" field
One solution could be to use intermediate model:
class Element(models.Model):
version_id = models.ManyToManyField(
'VersionRefbook',
through='ElementVersionRefbook'
)
value = models.CharField(max_length=300)
class ElementVersionRefbook(models.Model):
version = models.ForeignKey(VersionRefbook, on_delete=models.CASCADE)
element = models.ForeignKey(Element, on_delete=models.CASCADE)
code = models.CharField(max_length=100)
class Meta:
unique_together = ('version', 'code')
You can create instances as below:
elem = Element.objects.create(value='some value')
refbook = Refbook.objects.create(
code='some code',
name='some name',
description='some desc'
)
version = VersionRefbook.objects.create(
refbook_id=refbook,
version='1.0',
date='2023-01-01'
)
elem_version_refbook = ElementVersionRefbook.objects.create(
version=version,
element=elem,
code='123'
)
elem_version_refbook = ElementVersionRefbook.objects.create(
version=version,
element=elem,
code='345'
)
If you try to add ElementRefbook entry with same version and code as below, it will raise IntegrityError
elem_version_refbook = ElementVersionRefbook.objects.create(
version=version,
element=elem,
code='123'
)