This presentation is a part of Sunscrapers' weekly talks. Michał presents and evaluates different approaches to organising business logic code in a Django Project.

1. sunscrapers.comYour favored tech partner.
Thick QuerySets and Thin Models, or
Where to Put Business Logic in
Django
Michał Nakoneczny
2017-05-19

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A layer which encodes the real-world business rules that
determine how data can be created, displayed, stored, and
changed.
Business Logic Layer

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Idea 1: Fat Models

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A Company Blog
class Post(Model):
STATUS_DRAFT, STATUS_APPROVED, STATUS_PUBLISHED = range(3)
STATUS_CHOICES = (
(STATUS_DRAFT, _('Draft')),
(STATUS_APPROVED, _('Approved')),
(STATUS_PUBLISHED, _('Published'))
)
title = models.CharField(max_length=255)
text = models.TextField()
status = models.PositiveSmallIntegerField(
choices=STATUS_CHOICES,
default=STATUS_DRAFT
)

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BL: Approving and Publishing
class Post(Model):
...
def approve(self):
self.status = Post.STATUS_APPROVED
self.save()
def publish(self):
if self.status != Post.STATUS_APPROVED:
raise PostInvalidStatusError()
self.status = Post.STATUS_APPROVED
self.save()

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Pros:
● Testable;
● Readable;
● DRY?
Cons:
● Unmaintanable in a large code base;
● Leads to unoptimised solutions.
Idea 1: Fat Models

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Idea 2: Forms/Views/Serializers

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Idea 3: Services

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BL: Approving and Publishing
def approve_post(post):
post.status = Post.STATUS_APPROVED
post.save()
def publish_post(post):
if post.status != Post.STATUS_APPROVED:
raise PostInvalidStatusError()
post.status = Post.STATUS_APPROVED
post.save()

11. sunscrapers.comYour favored tech partner.
Pros:
● Testable;
● Readable?;
● DRY
Cons:
● Unstructured in a large code base;
● Can lead to unoptimised solutions.
Idea 3: Services

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Idea 4: QuerySets/Managers

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BL: Approving and Publishing
class PostQuerySet(QuerySet):
def approve(self):
return self.update(status=Post.STATUS_APPROVED)
def publish(self):
return self.filter(status=Post.STATUS_APPROVED).update(
status=Post.STATUS_PUBLISHED)
class Post(Model):
...
objects = PostQuerySet.as_manager()

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Pros:
● Testable;
● Readable;
● DRY;
● Optimised (unless you do something stupid).
Cons:
● Checking Business Logic rules is more complicated.
● Integrations get more difficult.
Idea 4: QuerySets/Managers

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Business Logic Rules
def publish(self):
return self.filter(status=Post.STATUS_APPROVED).update(
status=Post.STATUS_PUBLISHED)
def publish(self):
if self.exclude(status=Post.STATUS_APPROVED).exists():
raise PostInvalidStatusError()
return self.update(status=Post.STATUS_PUBLISHED)

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Integrations
def pay(self):
Posts_data = …
pay_service = PayService()
try:
pay_service.pay_batch(posts_data)
return self.update (status=Post.STATUS_PAID)
except …:
...

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Thanks!
Questions?