The document discusses using database constraints to improve reliability, speed, and security of a relational database. It recommends using simple constraints like not null, unique indices, and foreign keys to define the database interface. More complex constraints like polymorphic relationships can be handled by the Polymorpheus gem, which takes care of validation and uniqueness through migrations. The overall message is that properly setting up database constraints through tools like the gem can keep application logic clean while establishing a robust database structure.

1. Using database
constraints wisely
Barun Singh
bsingh@wegowise.com
Founder & CTO

2. What is a database?
A collection of data that you can
read, write, and organize
The software tools that let you do this

3. What do we want from a
database?
It should be …
• Reliable
• Fast
• Secure
• …

4. A relational database
is not the same as
a generic data store

5. A relational database
is not the same as
a generic data store

6. What do we want from a
relational database?
It should be …
• Reliable
• Fast
• Secure
• Sensible
• …

7. Wait a minute…
Is this Rails approved™ ?

8. The Rails way says…
• You should be database agnostic
• Application layer is the only really important
part

9. The database is important

10. But why?
Because you make mistakes.

11. What’s the worst that can
happen?
• Feature doesn’t work
• Site goes down
• Irrecoverable data loss

12. I write tests. Why bother
with database constraints?
I test my entire application flow through
integration tests.
Why write unit tests?

13. I write tests. Why bother
with database constraints?
My application requires Javascript, and I have
JS validations.
Why bother with model-level validations?

14. Your database has an
interface.

15. How?
• Simple constraints: [not null]
• Unique indices
• Foreign keys [foreigner gem]

16. It’s not always that simple

17. Polymorphism
Imageable
Employee
Picture
Product

18. Polymorphism
What does
this
reference?

19. Polymorphism

20. Polymorpheus

21. A couple little snags…
How can you make sure that one picture
has only an employee_id or product_id
but not both?
(xor constraints are tough)
And what if the polymorphic relationship has
to be unique?

22. Polymorpheus
The gem takes care of all of this for you
through a simple migration method

23. Polymorpheus
The gem takes care of all of this for you
through a simple migration method

24. Polymorpheus

25. Polymorpheus
Also provides a validation method,
model level validations are still important

26. You can keep your
application logic clean
and
set up your database
properly

27. Questions?
bsingh@wegowise.com