Ruby on Rails

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24

• Ruby on Rails

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• Convention is the ruler of all.
• Pindar
• Where the telescope ends, the microscope begins.
  Which of the two has the grander view?
• Victor Hugo
• We grow more partial for the observers sake.
• Alexander Pope

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• Those who cannot remember the past are condemned
  to repeat it.
• George Santayana
• Lets look at the record.
• Alfred Emanuel Smith
• All that matters is that the miraculousbecome
  the norm.
• Henry Miller

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OBJECTIVES

• In this chapter you will learn
• Basic Ruby programming.
• How to use the Rails framework.
• The Model-View-Controller paradigm.
• How to use ActiveRecord to model a database.
• How to construct web applications that
  interactwith a database.
• How to create a web-based message forum.
• How to develop Ajax-enabled applications in
  Rubyon Rails.
• How to use the built-in Script.aculo.us library
  toadd visual effects to your programs.

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• 24.1 Introduction
• 24.2 Ruby
• 24.3 Rails Framework
• 24.4 ActionController and ActionView
• 24.5 A Database-Driven Web Application
• 24.6 Case Study Message Forum
• 24.6.1 Logging In and Logging Out
• 24.6.2 Embellishing the Models
• 24.6.3 Generating Scaffold Code
• 24.6.4 Forum Controller and Forum Views
• 24.6.5 Message Controller and Message Views
• 24.6.6 Ajax-Enabled Rails Applications
• 24.7 Script.aculo.us
• 24.8 Wrap-Up
• 24.9 Web Resources

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24.1 Introduction

• Ruby on Rails (also known as RoR or just Rails)
  is a framework for developing data-driven web
  applications.
• A web framework is a set of libraries and useful
  tool that can be used to build dynamic web
  applications.
• Ruby on Rails is different from most other
  programming languages because it takes advantage
  of many conventions to reduce development time.
  If you follow these conventions, the Rails
  framework generates substantial functionality and
  perform many tasks for you.
• Ruby on Rails has built-in libraries for
  performing common web development tasks, such as
  interacting with a database, sending mass e-mails
  to clients or generating web services.
• Rails has built-in libraries that provide Ajax
  functionality, improving the user experience.
  Rails is quickly becoming a popular environment
  for web development.
• Ruby on Rails was created by David Heinemeier
  Hansson of the company 37Signals.

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24.2 Ruby

• The Ruby scripting language was developed by
  Yukihiro Matz Matsumoto in 1995 to be a
  flexible, object-oriented scripting language.
• Rubys syntax and conventions are intuitivethey
  attempt to mimic the way a developer thinks. Ruby
  is an interpreted language.
• Instant Rails is a stand-alone Rails development
  and testing environment that includes Ruby,
  Rails, MySQL, Apache, PHP and other components
  necessary to create and run Rails applications.
• If you are using Mac OS X, there is an
  application similar to Instant Rails called
  Locomotive.
• The method puts prints the text to the terminal,
  followed by a newline.
• A method can have parentheses surrounding its
  parameters, but this is not typical in Ruby
  unless they are used to avoid ambiguity.
• A line of Ruby code does not have to end with a
  semicolon, although one can be placed there.
• One way to run a Ruby script is to use the Ruby
  interpreter.
• IRB (Interactive Ruby) can be used to interpret
  Ruby code statement by statement.

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Fig. 24.1 Instant Rails application running.
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24.2 Ruby (Cont.)

• Ruby uses dynamic typing, which allows changes to
  a variables type at execution time.
• Everything is an object in Ruby, so you can call
  methods on any piece of data.
• Hash Objects are mapped to other Objects in
  key/value pairs.
• The exclamation point after a method name is a
  Ruby convention indicating that the object on
  which the method is called will be modified.
• Ruby has support for code blocksgroupings of
  Ruby statements that can be passed to a method as
  an argument.
• The initialize method acts like a constructor in
  other object-oriented languagesit is used to
  declare and initialize an objects data.
• When each instance of a class maintains its own
  copy of a variable, the variable is known as an
  instance variable and is declared in Ruby using
  the _at_ symbol.
• Classes can also have class variables, declared
  using the _at__at_ symbol, that are shared by all
  copies of a class.
• When an object is concatenated with a string, the
  objects to_s method is called to convert the
  object to its string representation.

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Outline
The puts command writes a line of text to the
console
welcome.rb
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Fig. 24.3 Launching the Ruby Interpreter in
Instant Rails.
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Fig. 24.4 Using the Ruby interpreter to run a
simple Ruby script.
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Fig. 24.5 Using Interactive Ruby to execute
Ruby statements.
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Outline
Ruby uses as the assignment operator
Enclosing a variable in curly braces ()preceded
by a pound sign () causes it to be interpolated
types.rb (1 of 2)
You can call the round method on any variable of
type Fixnum
Ruby variables are dynamically typed, so
variables have no explicit type, but can hold any
type of data at any time
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Outline
The capitalize method capitalizes the first
letter of a string
types.rb (2 of 2)
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Outline
Arrays can be created using comma-separated lists
in square brackets
arraysAndHashes.rb
The length of an array is stored in its length
property
Elements are accessed using square bracket
notation You can also access items starting from
the end of the array using negative indices
You can create a hash, which stores key-value
pairs, by separating keys from values with the gt
operator and enclosing the list in curly braces
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Outline
ControlStatements.rb
Methods are defined using the def keyword, and
method definitions end with the end keyword
The each method can be used on arrays to iterate
through their elements.
The parameters of a code block are placed between
pipe characters ()at the beginning of the block
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Outline
Classes are defined beginning with the class
keyword and ending with the end keyword
Classes.rb (1 of 2)
Class variables are preceded by _at__at_
Create a constructor by defining an initialize
method
Instance variables are preceded by _at_
Every class has a to_s method that returns a
string representation of the object. This class
overrides its to_s method
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Outline
Classes.rb (2 of 2)
Create an instance using the class name and new,
supplying any arguments required by the
constructor
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24.3 Rails Framework

• While users have benefitted from the rise of
  database-driven web applications, web developers
  have had to implement rich functionality with
  technology that was not designed for this
  purpose.
• The Rails framework combines the simplicity of
  development that has become associated with Ruby
  with the ability to rapidly develop
  database-driven web applications.
• Ruby on Rails is built on the philosophy of
  convention over configurationif you follow
  certain programming idioms, your applications
  will require little or no configuration and Rails
  will generate substantial portions of the
  applications for you.
• The Model-View-Controller (MVC) architectural
  pattern separates application data (contained in
  the model) from graphical presentation components
  (the view) and input-processing logic (the
  controller).
• ActiveRecord is used to map a database table to
  an object.
• ActionView is a set of helper methods to modify
  user interfaces.
• ActionController is a set of helper methods to
  create controllers.

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Fig. 24.10 Model-View-Controller architecture.
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Fig. 24.11 Rails directory structure for a new
Rails application.
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24.4 ActionController and ActionView

• Ruby on Rails has two classes, ActionController
  and ActionView, that work together to process a
  client request and render a view.
• To generate a controller in Rails, you can use
  the built-in Controller generator by typing ruby
  script/generate controller name.
• A Ruby on Rails application must be run from a
  web server
• Instant Rails comes with a built-in web server
  named Mongrel, which is easy to use to test Rails
  applications on the local machine.
• When generating output, a controller usually
  renders a templatean XHTML document with
  embedded Ruby that has the .rhtml filename
  extension.
• The request object contains the environment
  variables and other information for a web page.
• Erb (embedded Ruby) that is located between the
  lt gt tags in rhtml files is parsed as Ruby code
  and formatted as text.
• A set of Ruby tags without an equals signlt
  gtrepresents statements to execute as Ruby code
  but not formatted as text.
• Rails allows you to add headers and footers with
  a layouta master view that is displayed by every
  method in a controller.
• A layout can generate a template for a specific
  method using yield.

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Outline
app/controllers/welcome_controller.rb
Call the render method, specifying its parameter
using the text symbol
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Fig. 24.13 Starting the Mongrel web server.
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Outline
app/controllers/welcome_controller.rb
Define a class variable in the controller that
contains information about the server
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Outline
app/views/welcome/hello.rhtml
The view has access to the controllers class
variables
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Outline
app/views/layouts/welcome.rhtml
A controllers action_name method displays the
action that is currently being called
The layout yields to the view associated with the
current action
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Fig. 24.17 Creating a model in the Ruby
Console.
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24.5 A Database-Driven Web Application

• Rails makes extensive use of Object-Relational
  Mapping (ORM) that maps a database to application
  objects.
• The objects that Rails uses to encapsulate a
  database inherit from ActiveRecord.
• One ActiveRecord convention is that every model
  that extends ActiveRecordBase in an application
  represents a table in a database.
• By convention, the table that the model
  represents has a name which is the lowercase,
  pluralized form of the models name.
• Rails uses a generator to create the Employee
  model. You use a generator by typing ruby
  script/generate model employee in the Ruby
  Console, after navigating to your application
  directory.

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24.5 A Database-Driven Web Application (Cont.)

• The ActiveRecord object has a special feature
  called Migration, which allows you to perform
  database operations within Rails.
• ActiveRecord has built-in functionality for many
  create, retrieve, update and destroy methods
  known in Rails as CRUD.
• We can execute the migration using Rubys rake
  command by typing rake dbmigrate, which will
  call the self.up method of all the migrations
  located in your db/migrate directory.
• If you ever want to roll back the migrations, you
  can type rake dbmigrate VERSION0, which calls
  each migrations self.down method.
• The scaffold method is a powerful tool that
  automatically creates CRUD functionality. It
  creates methods such as new, edit and list so you
  dont have to create them yourself.

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Outline
The up method in a migration does the work on the
database
db/migrate/001_create_employees.rb
We create a table in the database with three
columns containing strings
Create three entries in the table, specifying
values for each field
The down method undoes what the up method did so
that you can roll changes forward and back
To undo the changes, we simply drop the table
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Common Programming Error 24.1

• If the code that comes after the creation of the
  table in the self.up is erroneous, the migration
  will fail, and will not be able to execute again
  because the table will already exist. Also, Rails
  will not have marked the migration as
  successfully completed, so the version will still
  be 0 and the migration cannot be rolled back. One
  way to prevent this problem is to force the table
  to be dropped every time before creating it.
  Another solution is splitting up the migration
  into smaller discrete migrations, one to
  createthe table and another to insert data in
  the table.

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Outline
employee.rb
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Outline
The scaffold method dynamically generates any
CRUD methods that are not already defined in the
controller
app/controllers/employees_controller.rb
Our list method (called when the list action is
invoked) creates an array of all employees so
that the view can display them
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Fig. 24.21 View of the new action when
generated by the scaffold.
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Outline
app/views/employees/list.rhtml
Loop through each employee in the database
Access each employees first and last name as
properties of the current employee object
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24.6 Case Study Message Form

• Validators that will be called when the database
  is modified, can be applied to an object that
  inherits from ActiveRecord.
• The method validates_presence_of ensures that all
  the fields specified by its parameters are not
  empty.
• The method validates_format_of matches all the
  fields specified by its parameters with a regular
  expression.
• The link_to method is used to link to an action
  in the controller and pass arguments to it.
• A partial is a block of HTML and embedded Ruby
  code stored in another file and inserted directly
  into the document.
• Rails includes a JavaScript library called
  Prototype that contains easy-to-use cross-browser
  Ajax functions.
• The javascript_include_tag helper method is used
  to link in JavaScript libraries.
• The link_to_remote method allows us to link to
  JavaScript that we included in the layout file.
• Specifying the url and update parameters inside
  the link_to_remote method tells Rails to convert
  these tags into prototype Ajax.Updater objects
  that will update the page asynchronously.

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Outline
Create a database table to store users
db/migrate/001_create_users.rb
Create a test user
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Common Programming Error 24.2

• Creating a column without explicitly specifying a
  limit on length will causeRails to truncate the
  data entered intothe database with
  database-defined limits.

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Outline
app/models/user.rb
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Outline
The admin action creates a new User
app/controllers/users_controller.rb
The validate action implements a login attempt
If the login is successful, store the User object
in a session variable
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Performance Tip 24.1

• Storing full objects in the session is
  inefficient. The user object is one of the rare
  exceptions, because it doesnt change very often
  and is frequently needed in web applications that
  manage the state information for unique clients.

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Outline
The login form submits to the validate action,
where the login information is processed
app/views/users/admin.rhtml
Both fields specify a model and a column to which
they correspond
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Outline
app/views/layouts/users.rhtml
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Outline
db/migrate/002_create_messages.rb
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Outline
Specify that each Message belongs to a Forum
app/models/message.rb
Validators make sure that a Message is not
created unless the title, author, email, and
message are defined, and the email is properly
formatted
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Outline
db/migrate/003_create_forums.rb
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Outline
app/models/forum.rb
A forum model is a container for multiple message
objects
When a forum is destroyed, all of its messages
are destroyed with it
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Outline
app/controllers/forums_controller.rb (1 of 2)
Ensure that anything modifying the database is
sent to the server as a POST request
Go back to the list page once a change has been
submitted
Use flash to display an error at the top of the
page if the user is not logged in
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Outline
app/controllers/forums_controller.rb (2 of 2)
Notify the user that their action was successful
Only allow the user to see a list of forums to
delete if the user is an administrator
Delete a forum and redirect to the list page
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Outline
Use the _at_forums array (defined in the controller)
to create a list of all
app/views/forums/list.rhtml (1 of 2)
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Outline
app/views/forums/list.rhtml (2 of 2)
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Outline
app/views/forums/new.rhtml
The render partial method inserts the contents
of a partial file (in this case _form.rhtml) into
a document
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Outline
app/views/forums/_form.rhtml
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Outline
app/views/forums/delete.rhtml
Use the collection_select method to generate a
drop-down menu from the _at_forms array.
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Outline
app/views/layouts/forums.rhtml (1 to 2)
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Outline
app/views/layouts/forums.rhtml (2 to 2)
Link to the admin action to allow the user to
login
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Outline
app/controllers/messages_controller.rb (1 to 2)
Get an array of messages belonging to the forum
with the forum_id stored in the session object
for the list action
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Outline
app/controllers/messages_controller.rb (2 to 2)
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Outline
app/views/messages/list.rhtml (1 to 2)
Format the creation time using the strftime
method of the Time object stored in
messagecreated_on
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Outline
app/views/messages/list.rhtml (2 to 2)
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Outline
app/views/messages/_form.rhtml (1 to 2)
The text_area method generates a textarea XHTML
element
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Outline
app/views/messages/_form.rhtml (2 to 2)
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Outline
app/views/layouts/messages.rhtml
The stylesheet_link_tag generates a link element
to an external style sheetin this case,
scaffold.cssin public/stylesheets/
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Outline
app/views/layouts/forums.rhtml (1 to 2)
Include the default JavaScript libraries,
including Script.aculo.us and Prototype
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Outline
app/views/layouts/forums.rhtml (2 to 2)
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Outline
app/views/forums/list.rhtml (1 to 2)
The link to each forum makes an Ajax call,
displaying the selected forums messages in the
currentForum div using a partial page update
Messages are displayed in this div
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Outline
app/views/forums/list.rhtml (2 to 2)
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Outline
app/views/messages/list.rhtml (1 to 2)
The New Message link puts the form in on the same
page using a partial page update
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Outline
app/views/messages/list.rhtml (2 to 2)
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Outline
Forum creation is also done without reloading the
entire page
app/views/messages/new.rhtml (1 to 2)
Canceling forum creation replaces the form with
the list of forums
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Outline
app/views/messages/new.rhtml (2 to 2)
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24.7 Script.aculo.us

• Script.aculo.us allows you to easily create
  visual effects similar to those in Adobe Flash
  and Microsoft Silverlight.
• The library provides many pre-defined effects, as
  well as the ability to create your own effects
  from the pre-defined ones.
• The Script.aculo.us library also provides
  drag-and-drop capability through the
  draggable_element and drop_receiving_element
  methods.
• The sortable_element method allows you to
  describe a list that allows the user to drag and
  drop list items to reorder them.
• Script.aculo.us also provides the
  text_field_with_auto_complete method, which
  enables server-side autocompletion of a text
  field.

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Fig. 24.46 Script.aculo.uss Fade effect.
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Outline
app/views/scriptaculous_demo/index.rhtml
Play a Shrink effect on the image div before
proceeding to the playEffect action
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Outline
app/controllers/scriptaculous_demo_controller.r
b
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Outline
app/views/layouts/application.rhtml
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Outline
app/views/scriptaculous_demo/_link.rhtml (1 to
5)
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Outline
app/views/scriptaculous_demo/_link.rhtml (2 to
5)
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Outline
app/views/scriptaculous_demo/_link.rhtml (3 to
5)
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Outline
app/views/scriptaculous_demo/_link.rhtml (4 to
5)
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Outline
app/views/scriptaculous_demo/_link.rhtml (5 to
5)
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Fig. 24.51 Flickr Photo Viewer showing search
results for bugs.
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Outline
app/view/flickr/index.rhtml (1 to 2)
A form that searches for tags, using blind
effects and a loading indicator
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Outline
app/view/flickr/index.rhtml (2 to 2)
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Outline
app/controllers/flickr_controller.rb
Search for tags and display paginated results
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Outline
app/views/flickr/_thumbs.rhtml
Display a thumbnail that links to the full sized
image
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Outline
app/views/flickr/fullsizeImage.rhtml
Display the full size version of an image