oo and rails

OOPOOPWhat? Why?What? Why?

on Rails?on Rails?

\\

earliest evidence of double-entry bookkeeping earliest evidence of double-entry bookkeeping is the Farolfi ledger of 1299-1300.is the Farolfi ledger of 1299-1300.

The earliest evidence of

full double-entry bookkeeping

is the Farolfi ledger of 1299-1300.

Why am I talking about this?

The earliest evidence of

full double-entry bookkeeping

is the Farolfi ledger of 1299-1300.

Why am I talking about this?

...because we are amazingly lucky that our idea creators are still alive!

Barbara Liskov

ACM Turing Award lecture (2009) 11:50http://www.infoq.com/presentations/liskov-power-of-abstraction

The entire system should be broken up into

“Partitions”

Each has access to some hidden state

Each provides operations

Only interact by calling each others operations

Why doesn't she get to the point?

She's talking about 'Objects'!

Why doesn't she get to the point?

She's talking about 'Objects'!

Object Oriented Programming didn't exist...

Who invented OOP?

Alan Kay is credited with coining the term

Led research to develop SmallTalk

=> 1972 (40ish years ago)

Who invented OOP?

It developed later as the dominant methodology

when programming languages supporting

the techniques became widely available

(C++, Java, Delphi)

=> early-mid 1990s (20ish years ago)

"Actually I made up the term "object-oriented", and I can tell you I did not have C++ in mind."

-- Alan Kay

I thought of objects

being like biological cells

and/or individual computers on a network,

only able to communicate with messages

(so messaging came at the very beginning)

– Alan Kay

Design is *all* about Dependencies

If you refer to something

---> you depend on it

When the things you depend on change

---> you must change

-- Sandi Metz, "Solid Design" GoRuCo 2009

A----\

B-----\

C -----> X

D-----/

E ---/

● if x changes everybody has to change!● x better not change! It should be stable

/--> A

/---> B

Z ------> C

\---> D

\--> E

● if z changes no body cares!● if c changes, only z may have to● z is isolated!

J K L→ →

● if L changes, K may have to● Maybe J will too?

OO Design in Rails?

ActiveRecord / Objects?

Rather than asking for data,

tell a class to do something for you

ActiveRecord / Objects?

Procedural gets info, then makes decision

ActiveRecord / Objects?

Procedural gets info, then makes decision

Object Oriented code tells objects to do things

ActiveRecord / Objects?

Procedural gets info, then makes decision

Object Oriented code tells objects to do things

What do you do in Rails code?

S.O.L.I.D

S: SRP: Single Responsibility Principle

O: OCP: Open/Closed Principle

L: LSP: Liskov Substitution Principle

I: ISP: Interface Segregation Principle

D: DIP: Dependency Inversion Principle

Single Responsibility

An object should have only a single responsibility

One and ONLY ONE reason to change

If you have to use the word “AND” or “OR”to describe your class....

Jim Weirich talk on SOLID

Jim asked: AR mixes domain + persistence -- is it ok?

Jim Weirich talk on SOLID

Jim asked: AR mixes domain + persistence -- is it ok?

Response: ● NO! As design gets complex and interactions

between objects get complex, it confuses things.

● NO! It's harder to test.

Jim Weirich talk on SOLID

Jim asked: AR mixes domain + persistence -- is it ok?

Response:● NO! As design gets complex and interactions

between objects get complex, it confuses things.● NO! It's harder to test.

Jim said: Maybe it IS OK in your case...!● IT'S PRINCIPLES NOT RULES! up to you!

The DHH Factor

...later on twitter

@paulc: ... What are your thoughts on separating logic + persistence?

The DHH Factor

...later on twitter

@paulc: ... What are your thoughts on separating logic + persistence?

@dhh: Fuck. That. Shit. Same complete wank. "Rails is not your application".

If you're building a web app, of course it is.

#find and the infinite protocol

If ActiveRecord models

Are your Domain Models

You are exposing an infinite protocol!

.where(:category => “Blah”).order(:name)

Interface Segregation Principle

Code should depend on as NARROW protocol as possible

The protocol is the messages between objects

Not WIDE interfaces!

It's just encapsulation, right?

#find and the infinite protocol

If ActiveRecord models

Are your Domain Models

You are exposing an infinite protocol!

.find(:first)

Single Responsibility

Build models without referencing ActiveRecord?

Why?● Faster tests● Separate concerns for complex situations● Expose a clean, domain-oriented interface● Logic unrelated to persistence:

At the intersection of persistable objectsYou often find business rules. IT'S A THIRD PLAIN RUBY OBJECT!

Single Responsibility

Build models without referencing ActiveRecord?

How?● FigLeaf approach?

– The gem “hides” all AR methods (so you can't access them outside your model)

– You expose the functionality you mean

– It's domain driven

Single Responsibility

Build models without referencing ActiveRecord?

How?● Domain Object approach?

– You keep AR models as thin as possible

– Add a separate object for Domain logicthat accesses the AR model

– Everybody else accesses the Domain object

...thoughts

“There is no problem in computer science

that cannot be solved by adding another layer of indirection,

except having too many layers of indirection”

...thoughts

@dhh

I hate the disconnect between design patterns/abstract ideas and implementation

Your basic question should be “Is the code better?”

...thoughts

@avdi

You absolutely MUST not give up the fun you found when first coming to ruby when working on larger systems

...your thoughts?