Decouple Django apps with business logic to have better reusability.

1. Concepts in Django
Douwe van der Meij
Goldmund, Wyldebeast & Wunderliebe
vandermeij@gw20e.com
github.com/douwevandermeij

2. Project
INSTALLED_APPS = (
...
'crm',
'webshop',
...
)

3. Dependencies
● Inter-app foreign
key constraint
crm
Customer
Project
account_nr
● How to reuse
webshop without
crm?
Order
order_nr
webshop
OrderLine

4. Concepts
Define:
● Producers
○ Contain data
Producer
● Consumers
○ Don’t contain data
○ Need a producer
Consumer

5. Customer concept
The concept we’re talking about:
● A customer
○ Has a number

6. Applications with concepts
crm
Producer
Customer
Project
account_nr
webshop
Consumer
Customer
number
Order
OrderLine

7. Define Producers
Conceptname
CONCEPTS = {
Producer model
'CustomerConcept': {
'crm.models.Customer': {
'number': 'account_nr',
Field mapping
},
},
}
● Map to existing models

8. Consume
class Order(models.Model):
customer = models.ForeignKey(CustomerConcept)
order_nr = models.IntegerField()

9. Dependencies
crm
● ORM level
Customer
Customer
CustomerConcept
concepts
Order
webshop
Project

10. Dependencies
crm
Customer
Order
webshop
Project
● Database level

11. The magic (1/4)
from django.conf import settings
for concept_name, mapping in settings.CONCEPTS.items():
for klass, fields in mapping.items():
...
● The concepts app
○ Loops over settings.CONCEPTS

12. The magic (2/4)
from concepts.util import load_class
● Create a dynamic class
● With
in
...
concept = type(
proxy = True
Meta
concept_name,
(load_class(klass),),
{
'__module__': __name__,
'Meta': type('Meta', (), {'proxy': True}),
},
)
...

13. The magic (3/4)
...
for _property, _field in fields.items():
if not hasattr(concept, _property):
setattr(
concept,
_property,
property(
lambda self: getattr(self, _field),
lambda self, value:
setattr(self, _field, value),
),
)
...
● Create properties for
all fields

14. The magic (4/4)
import concepts
● Bind the concept to
the concepts module
...
setattr(concepts, concept_name, concept)

15. Conclusion
● Concepts make apps reusable
○ Without dependencies
● The only dependency
○ Is a concept
● The concept producer
○ You may choose yourself
○ On existing apps

16. Try it yourself
● github.com /
douwevandermeij /
concepts_example
● Related work
○ Django Swappable Models
■ Like swapping auth.User model in 1.5+
○ Zope Component Architecture
■ Interfaces: implement/adapt

17. Thank you!
Douwe van der Meij
Goldmund, Wyldebeast & Wunderliebe
vandermeij@gw20e.com
github.com/douwevandermeij