Transcript
Rails
28-Apr-16
What is Rails?
Rails is a framework for building web applications This involves:
Getting information from the user (client), using HTML forms Doing validation Maintaining session information Managing a database Displaying results to the user
Rails differs from similar frameworks in C++ or Java
Rails applications can be built much more quickly Rails requires knowledge of fewer technologies
Required software
You need:
The Ruby language A database, such as MySQL A Ruby-capable web server, such as lightppd, WEBRick, or Mongrel
On Windows, InstantRails provides all of these On Macintosh, Locomotive provides these On Linux you probably have to assemble the pieces yourself It’s also helpful to have:
A good text editor, such as TextMate or TextPad A Ruby IDE, such as Eclipse with the RDT plugin, or RadRails A GUI interface for looking at your database
Rails philosophy
Convention over configuration
Don't Repeat Yourself (DRY)
Other frameworks use several XML configuration files to specify where everything is, and how the parts are related Rails assumes a standard configuration. Every piece of information (program or data) should be represented once and only once Rails provides a structure that encourages DRY
Agile development
In Rails we start with a working program and “grow” it by making small changes In Rails it is easy to re-test after every small change Rails provides strong support for unit testing
Three environments
By default, Rails projects use three environments The development environment
The test environment
Classes are reloaded after every change, so every change you make happens “immediately”--you don’t have to restart the server This is where you do most of your work
Classes are also reloaded after every change For each test, Rails creates a fresh copy of the data in the test database
The production environment
Classes are only loaded once Changes typically require stopping and restarting the server The environment is tuned for speed
Scripts
A lot of work is done in the Rails world by generate and rake scripts The generate scripts are called like this: ruby script\generate generator options args
Generators typically create files and directories for you Some built-in generators are: controller, model, scaffold, mailer, and web_service
The rake (like UNIX make) scripts are called like this: rake name:name:name
Rake scripts are typically used to propagate changes by updating files that depend on other files Rake scripts are written in Ruby
Starting an application
To create a new application, say, “MyApplication”, enter the command: rails MyApplication
This creates a directory (folder) named MyApplication and, beneath it, a large directory structure and a number of files
Creating an empty database
The following instructions are for MySQL on Windows, as configured in InstantRails 1. At the command line, enter mysql -u root –p
and just hit Enter when prompted for a password 2. Type the following lines into MySQL: •
create database MyApp_development; grant all on MyApp_development.* to 'ODBC'@'localhost'; The above creates the development database for an application named MyApp; you can repeat these two lines to create the test and production databases, if you like
3. Type exit
The directories, I
Rails creates the following subdirectories of the MyApplication directory:
app/ -- more about this directory later components/ -- reusable components config/ -- configuration information, including database connection parameters db/ -- database schema information doc/ -- autogenerated documentation lib/ -- code produced by your company and shared by many applications vendor/ -- purchased code shared by many applications
The directories, II
More subdirectories
log/ -- log files produced by the application public/ -- the web-accessible directory; your program appears to be running from here Rakefile/ -- scripts for creating documentation and tests script/ -- utility scripts tests/ -- unit tests, functional tests, mocks, and fixtures
Of these, the app subdirectory is the most important, but you will probably also use the config, db, and test directories
The app subdirectory
The app/ subdirectory contains
The controllers/ subdirectory, which contains
The helpers/ subdirectory, which contains
The application_helper.rb file A table_helper.rb file for each table in your application
The models/ subdirectory, which contains
The application.rb file A table_controller.rb file for each table in your application
A table.rb file for each table in your application
The views/ subdirectory, which contains
The index.rhtml file action.rhtml files for most of the methods in your controllers
Naming conventions in Ruby
Ruby requires the following naming conventions:
Ruby encourages the following naming conventions:
The names of classes, modules, and constants must begin with a capital letter The names of variables and methods must begin with a lowercase letter The names of classes, modules, and constants should be written in CamelCase; for example, ActiveRecord The names of variables and methods should be written in all lowercase, with underscores between words; for example, chunky_bacon
File names in Ruby follow the same rules as variable and function names; for example, my_application.rb
“Convention over configuration”
In other web application frameworks, you need many configuration files to specify how the various parts of your application fit together Rails assumes you have named things in a certain way, then finds the parts and fits them together itself Rails uses both capitalization and pluralization rules
Model-View-Controller (MVC)
Rails uses the MVC design pattern In somewhat oversimplified terms,
The Model does all the “business logic,” or computation The View displays results to the user The Controller ties it all together
It gets input from the user It takes actions depending on what the user wants done; this typically involves asking the Model to do some work, then collecting the results of that work It tells the View to display those results
Naming conventions in Rails, I
The Model:
The tables have lowercased, underscored, pluralized names A class that describes a record in a table has a CamelCase singular name A file containing the class has a lowercased, underscored, singular name
The View:
The URL is lowercased, underscored, and singular, and has the form http://...app/controller/method/parameters The file is lowercased, underscored, and singular, and has the form app/views/controller/method.rhtml The layout files in app/views/layout follow the same file naming conventions The helper module is CamelCase and singular The helper file is lowercased, underscored, and singular, and named app/helpers/application_helper.rb
Naming conventions in Rails, II
The Controller:
The URL is lowercased, underscored, and singular, and has the form http://...app/controller/method/parameters The controller class is CamelCase and singular The file is lowercased, underscored, and singular, and has the form app/controllers/table_controller.rhtml The controller’s methods are lowercased and underscored, and should typically be verbs
This sounds like a lot of conventions to learn, but in reality Rails creates most of these for you
Typical flow of control
A request from the browser goes to a controller in app/controllers/ The controller talks to the model in app/models/ (initially, by means of a scaffold) The model sends requests to the database (in db/) The database returns information to the model The model manipulates the information and returns it to the controller The controller uses a .rhtml template in app/views/ to send a response to the browser
Scaffolding
You can use “scaffolds” to build models and controllers for your application
A scaffold is an initial set of files which you will modify to do the work of your application The script ruby script\generate scaffold model controller will create the files app/models/model.rb and app/controllers/controller_controller.rb, among others
scaffold adds these methods: index, list, show, destroy, new, create, edit, update To learn more about scaffold, see ActionController::Scaffolding::ClassMethods
The browser call
The user talks to the application from a browser A request from the browser goes to a method in a controller Make the browser request with
http://host:port/ClassName/method/parameters If method is omitted, the default is index method must be defined in class_name_controller.rb The parameters are often blank
The controller
A request from the browser goes to a method in a controller
Create the controller with ruby script/generate controller ClassName (use \ instead of / on Windows) The controller will be app/controllers/class_name_controller.rb The controller extends ActionController::Base
Parameters sent to the URL (for example, by appending ?key=value pairs), are available by calling the params method If xyz is a key, its value can be accessed with either params['xyz'] or params[:xyz] A controller method must end by either:
Passing control to another controller method, with redirect_to :action => "some_other_method", or Passing control to a view template, either
Explicitly by ending with render :action => "some_template", or Implicitly, by not specifying a render action; this will render the template with the same name as the method
The model
If you are going to use a database (most Rails applications do), create the database first Create the model with ruby script/generate model ClassName The model extends ActiveRecord::Base The easiest things to add to the model are validations
Validations check whether a requested database entry or update is valid
A number of validation methods are defined in ActiveRecord::Validations::Base
Some useful validations
validates_presence_of(*attr_names) Tests that the attributes are not blank validates_numericality_of(*attr_names) Tests that the values of the attributes are numeric validates_length_of(attr_name, restrictions) Tests that the attribute's value satisfies the size restrictions. Some restrictions are: :minimum=>number, :maximum=>number, :is=>number, :within=>range validates_uniqueness_of(*attr_names) Tests that the attribute values are unique
validates_inclusion_of(attr_name, :in=>enumerable_object) Tests that the attribute's value is in the enumeration validates_each(*attrs) {|record, attr, value| ...} Tests each attribute with the code in the block
The view(s)
Views are written as HTML templates, in files with the .rhtml extension The RHTML file uses ERb, embedded Ruby:
Ruby statements (code to be executed, but not included in the HTML page) are enclosed in <% %> Ruby values, to be included in the resultant HTML, are enclosed in <%= %>
Some available methods are: start_form_tag, submit_tag, render, link_to, and end_form_tag
Components of a view
A view may consist of three kinds of parts: the main view, a layout, and some partials Layout Main view Partial
Constructing a view
Here’s what happens when you browse to http://localhost:3000/application/noun/verb Rails calls the verb method in controllers/noun_controller.rb. 1. When the controller's verb method finishes, Rails looks for a template named views/noun/verb.rhtml 2. Next, Rails looks for a template “layout” file named views/layouts/noun.rhtml •
If it can't find this file, it looks for a layout file named views/layouts/application.rhtml.
3. If no layout file is found, Rails uses the views/noun/verb.rhtml template “as is” 4. If a layout file is found, Rails inserts the verb.rhtml into the layout file, in the location marked <%= yield %>, and uses this combination to return to the user 5. Partials in the verb.rhtml file, indicated by <%= render :partial => 'form' %> , are filled in
Methods useful in the view
params Returns a list of parameters, as given to the controller
render(:action => partial) Renders the given partial template within the current template link_to(name, options = { }, html_options = nil) Returns the HTML for an link.
name is the text to be displayed. options are a hash of values to be used in creating the URL. html_options are a hash of key/value pairs to be added to the tag
One useful Rails-specific pair is :confirm => ' question?'
h(string) Replaces &, ", <, and > with &, ", <, and >, respectively. (This is a Ruby, not Rails, method.) form_tag, submit_tag, text_area_tag, radio_button_tag, etc. Rails methods for creating HTML tags (but you can write the tags directly). stylesheet_link_tag(name) Causes the HTML to use the stylesheet in public/stylesheets/name.css
Where methods come from
Controllers extend ActionController::Base Models extend ActiveRecord::Base All the relevant methods should be documented somewhere in http://api.rubyonrails.com/
Modules (mixins) make this more complicated, because a class can “mix in” methods from a variety of modules
Bad: The documentation isn’t complete (where is the params method?) Bad: The documentation doesn’t say what version it’s for Good: The source code is immediately available
For example, validates_numericality_of is in module ActiveRecord::Validations::ClassMethods
Yes, Rails is easy, but only after you learn your way around in it
Adding tables to the database
Rails cannot create a database; that has to be done in SQL
You can create a file (say, create.sql) containing SQL commands to create some tables, then execute mysql MyApp_development 1) do
4. Finally, enter the command rake db:migrate This creates a schema_info table in the database to tell SQL that this is the first version
Modifying the database
To make changes to the database, first run the script ruby script\generate migration SomeMeaningfulName
This creates a file named xxx_some_meaningful_name.rb, where xxx is the next version number The file will look like this: class SomeMeaningfulName < ActiveRecord::Migration def self.up end def self.down end end Edit this file to make the changes
The up method should make the changes, the down method should undo them
To “migrate” these changes to the actual database, run the script rake db:migrate
Methods to use in up and down
add_column(table_name, column_name, type, options = { })
Adds a new column to the named table
remove_column(table_name, column_name)
Removes a column from the database
rename_column(table, column, new_column_name)
The type may be :primary_key, :string, :text, :integer, :float, :decimal, :datetime, :timestamp, :time, :date, :binary, or :boolean The options may include :limit, :default, :null, :precision, or :scale
Renames a column
Other changes are most easily made by removing a column and adding it again with a different type or options
Status
This talk is based on a paper I’m writing, A Concise Guide to Rails, at http://alliance.seas.upenn.edu/~cis700/ruby/conciserails.html I’m still learning my way around Rails myself
I may have some things wrong I know I’m missing some important things
I would very much like to hear your corrections and suggestions
A small amount of extra credit is available
The End
