MVC Demystified 1

MVC Demystified

Webvisions 2007Michael P. Jones

michael@codeinmotion.com

Understanding the Essence of Ruby on Rails

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About Session

• Web applications• MVC• Pix Patisserie• MVC Another Look• Rails Essence• More Rails• Rails Testing• Other MVC

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About Speaker

• Past– Educational and Reservation software. (Java)– Founded Code in Motion. http://codeinmotion.com/

• Present– Creating custom Ruby on Rails applications.

• Recovery and prevention.• Screening and Interview• PDF generation of applications• Widgets

– Tailoring CMS Solutions• Radiant, Rails based CMS with extensions for business needs of the

client.

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Traditional Applications

• Browser directly accesses page.– Does not centralize control– No content/style separation– Easy and fast to produce– Difficult to maintain

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MVC Applications

• Browser accesses a “controller”– Centralizes control– Clean separation of content/style– More involved to produce– Easier to maintain and expand

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MVC

• MVC is an Architectural Design Pattern• Separates a web application into three different

modules.

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Design Pattern

• A pattern that has been developed to help programmers cope with common problems.

• Blueprints on how to construct something.

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MVC Means

• Most Vexing Conundrum (Amy Hoy)• Model | View | Controller

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Trip to Pix

• A Patisserie in Portland

http://qwendy.typepad.com/

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Typical Bakery Interaction

• Request a tasty treat from the baker

http://www.pixpatisserie.com/

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Baker Gathers Ingredients

• Baker gathers raw ingredients to fulfill the request.

• Some requests utilize same ingredients.

http://www.flickr.com/photos/moria/92792777/

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Baker Select Pan

• The pan dictates what the response looks like.

http://www.flickr.com/photos/tracyhunter/113563802/

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Baker responds with your treat

http://www.pixpatisserie.com/

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Pix Flow

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MVC Diagram

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Controller (Baker)

• Dispatches Requests and controls flow.• Centralizes access.• Interacts with Model and View.

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Model (Ingredients)

• Data representation and business logic.• Can be database/xml/etc• Business Logic• Examples:

– User– Bike– Question

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View (Pan)

• Data presentation and user input.• Renders the Model in to a View.• Can be HTML/PDF/WML/Javascript• No computations, very little logic, display logic i.e. loops

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MVC Diagram

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MVC Advantages

• Separation of interests.– Model centralizes business logic.– View centralizes display logic.– Controller centralizes application flow.

• Clean separation of content/style.• Improved decoupling.• Easier testing.• Allow multiple people to work on different parts.

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Rails and MVC

• Rails in an MVC framework• ”…Rails is such a framework that tries to remove the

complexity and drudgery of MVC, while still allowing you to realize all the benefits.” - DHH (David Heinemeier Hansson)

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Model: ActiveRecord

• ActiveRecord is a design pattern. (Martin Fowler)• Object wraps a row in the database.• Encapsulates data access.• Contains business logic.

• Handles relationships.• Handles validation.

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ActiveRecord Code

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View: ActionView

• Renders the view.• Both .rhtml and .rxml files.• Provides master layouts.• Uses ERb (embedded ruby) for templating and control.

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ActionView Code

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Controller: ActionController

• Controls application flow.• Controls which view to use.• Makes data available as instance variables @var• Contains several actions/defs• Controls rendering and redirection.

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ActionController Code

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All Together Now

http://wiki.rubyonrails.org/

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How it works

• your_domain.com/controller/action/id• Controller has actions.• View directory has directory for each

controller.• Same action name as .rhtml file• your_domain.com/bikes/edit/2

– Controller: bikes– Action: edit– Id: 2

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Rails Concepts

• DRY (Don’t Repeat Yourself)• Convention of configuration• Generators script/generate generator_name • :symbols or ‘symbols’

– represents names and strings

• YAML machine parsable human readable.

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More ActiveRecord

• Automatically maps to a table– No XML configs– Each row is an Object

• Several databases supported.• Provides

– CRUD: Create, Read, Update, Destroy– Getters/Setters– Validation: validates_presence_of, validates_format_of– Callbacks: hooks into lifecycle, trigger events before/after– Finding: Person.find(:all, :conditions => [ "category IN (?)",

categories], :limit => 50)

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More ActiveRecord Code

• Callbacks• Relations• Validation

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More ActionView

• Layouts– app/view/layouts/application.rhtml

• Partials– _partial_view.rhtml – Reuse of common view elements

• Helpers– Helper module per Controller– Contains Ruby code

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More ActionView Code

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More ActionController

• Filters– before, after and around processing hooks

• Routes– Replaces apache mod_rewrite– config/routes.rb

• Caching– Has caching to improve performance

• page• action• fragment

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More ActionController Image

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Pitfalls: Excess

• Model: God Object (anti-pattern)– Too much logic in a Model, one Model too powerful.

• View: Too much logic in View• Controller: Anemic Domain Model (anti-pattern)

– Too much business logic in Controller.

• Cargo cult programming – (over-)applying a design principle blindly without

understanding the reasons. - (wikipedia)

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Testing Rails

• Rails has great testing framework ‘baked in’ the framework– Test stubs created when code is ‘generated’.

• Unit to test models. – test/unit

• Functional to test views/controllers– test/functional

• Fixtures– Supply test data (yaml)

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Unit Testing

• Separate ‘test’ database is used.

• Fixtures supply data• Variety of ‘assert’

statements.

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Functional Testing

• Fixture• Setup• Tests

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Rake

• Rake is your friend.– Database tasks– Deployment– Documentation– Testing– Cleanup

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MVC Around The Web

• Java– Struts, Spring

• .NET– MonoRail (inspired by Rails)

• Perl– Catalyst, MayPole

• Php– CakePHP, Code Igniter

• Python– Django, TurboGears

• Ruby– Nitro, Wee

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Questions?

• MVC• Rails• Pix