To Lecture Notes

IT 231 -- 7/25/12

Review Questions

1. Explain the difference between the Rails helper functions yield and render.
   Ans: yield embeds a view within a layout page; render inserts code from another file into a view. A file containing code to be inserted with render must be preceded by a leading underscore character. For example, <%= render 'page' %> will insert the contents of the file _page.html.erb.
    
2. What is the filename containg the code that the following render command inserts?
   

   <%= render 'page' %> 
   Ans: _page.html.erb
   

3. How do you change the years displayed in a dropdown box for specifying a date on the form _form.html.erb?
   Ans: This answer is shown for the MovieReviewSite Example. Change Line 24 from
   

   <%= f.date_select :movie_release_date %>
   

   to

   <%= f.date_select :movie_release_date,
      :start_year => 1990, :end_year => 2012 %>
   

4. How do you specify these validations?
    
   
   1. The field :owner must be nonempty. Ans:
      

      validates :owner, :presence => true
      

   2. The field :password must have a minimum length of 8. Ans:
      

      validates :password, :length => { :minimum => 8 }
      

   3. The field :number_of_guests must be a number that is less than or equal to 4. Ans:
      

      validates :number_of_guests, :numericality => 
         { :less_than_or_equal_to => 4, :only_integer => true }
      

5. What does the Rails database console do for you? How can you invoke it?
   Ans: Use the Rails database console to issue SQL commands. Invoke the database console from the DOS Command Prompt.
   

   rails dbconsole
   

6. Name are some important HTTP commands?
   Ans: GET, HEAD, POST, PUT, DELETE.
    
7. Explain the difference between an HTTP GET request and an HTTP POST request.
   Ans: A GET request passes data back to the server in the URL; a POST request passes back the data in the head section of the page. Rails only uses POST to send information back the server.
    
8. What does CRUD mean?
   Ans: It is an acronym for the usual database operations: Create, Read, Update, and Destroy.
    
9. What does REST mean for describing server-side web processing?
   Ans: REST means Representational State Transfer. It means that
   
   1. A client-server architecture is used.
   2. Stateless communication. This means that neither the controller or the views remember the information on previously submitted views.
   3. Only the HTTP commands GET, POST, PUT, and DELETE are used to communicate with the server:
      

      CRUD Term	HTTP Command
      Create	POST
      Read	GET
      Update	PUT
      Destroy	DELETE

      
      
   
    
10. How do you change the port number to 3726 for the WEBrick server? Ans:
    

    rails server --port=3726
    

11. What is the range of numbers for the for well-known ports?
    Ans: 0 to 1023
     
12. Write a Hello World Ruby program. Ans:
    

    print "Hello, World."
    

13. Write a Ruby assignment statement that assigns 256 to the grand total. Ans:
    

    grand_total = 256
    

14. List six ways of executing Ruby statements? Ans:
     
    
    1. Using Interactive Ruby (IRB).
        
    2. As a Ruby batch script (ruby filename.rb).
        
    3. Using the Rails console (rails console).
        
    4. Loading a Ruby file to run in the Rails console (load filename.rb).
        
    5. Running Ruby code in the controller.
        
    6. Using Embedded Ruby (.erb file) to run the Ruby code in a view.

Routing

• Routes are specified in the file app/config/routes.rb.

• So far, the Rails applications we have created have used named routes.

• For example, if the controller name is Info and the view name is staff, then the named route used to view this page is

  get "info/staff"
  

• A project created using a scaffold uses a resource based route:

  resources :movie_reviews
  

• Here are some Rails helper methods generated by the Rails scaffold software from the student table for use by the link_to method:
   
  

  Action	Helper Method that Generates a URL path
  index	students_path
  show	student*
  new	new_student_path
  edit	edit_student_path
  destroy	student*

  
  *For the show and destroy actions, a helper method is not needed. The object variable student is passed directly to the link_to method.
   
• Here are some actual links from the Student example:
  

  <%= link_to 'Show', student %>
  
  <%= link_to 'Back', students_path %>
  
  <%= link_to 'New Student', new_student_path %>
  
  <%= link_to 'Edit', edit_student_path(student) %>
  
  <%= link_to 'Destroy', student, 
                  :confirm => 'Are you sure?',
                  :method => :delete %>
  

Ruby Details

• Constants and Variables
   
• Operators
   
• To modify command prompt Ruby code to display output on a Rails view: change
  

  c = 20
  f = 9 * c / 5 + 32
  puts f
  

  to

  c = 20
  @f = 9 * c / 5 + 32
  

  This code is placed into the controller method corresponding to the view that will contain the output. Then put this ERB code on the view page:
  

  <%= @f %>
  

Classes and Objects

• An object contains data values, called instance variables, and methods for getting things done.
   
• In a Ruby class, the data values are represented by instance variables, prefixed by @.
   
• Instance variables are valid anywhere in a Ruby script file, the Rails controller, or in ERB code on a view.
   
• Methods are also called subroutines, procedures, or functions.
   
• Methods are defined with the keyword def.
   
• Here is an example of a Ruby method from the Library Example (Example 7):
  

  def staff
     @title = "Staff Page"
  end
  

• Warm up Exercise for Classes:   Think of plausible instance variables for these two classes:
   
  
  BasketballPlayer     Coin
  
  Ans: Here are possible UML diagrams the BasketballPlayer and Coin classes.
   
• A class is the design or blueprint for an object.
   
• Here is a Miller analogy:
  

  object : class :: cookie :: cookie-cutter
  

• A UML class diagram is a concise way to represent a class definition.
   
• UML means Unified Modeling Language. It is a unification of the several object software modeling methodologies that existed in in 1994 when this unification project was started.
   
• Here are some UML diagrams of Ruby classes.
   
• A UML class diagram has three sections: (1) the class name, (2) the instance variables, (3) the methods. For example, in the String UML Diagram, the class name is String, there are no instance variables shown, and 24 methods are listed in the third section, starting with concat and ending with <=>.
   
• This is an example line from the UML diagram for the String class, and explanations of its contents:
  

  + insert(Fixnum, String) : String
  

  1. The + sign means that the method is public to be used anywhere in the Ruby script or Rails project.
      
     
  2. insert is the name of the method.
      
     
  3. Fixnum and String are the types of the input parameters to be passed in to the method when it is invoked.
      
     
  4. : String means that the method returns a String value.
  
  
• Write a Ruby program to test some of the methods of the String class. Here is a test file that includes the tests covered today (7/25) and also last time (7.23): test-string.rb. Also see the TestString Example (Example 15).
   
• Look at the Dice Example (Example 16), with test code, and the VendingMachine class in Project 3 for examples of Ruby classes.

More about Ruby

• Control Structures
   
• Look at the Ruby examples 17 through 21 on Control Stuctures.

• Look at Project 3.
   
• Here are some example statements that we discussed in class that are used to test the Time class:
  

  # Ssmple statements to use in your
  # test-time.rb file for Project 3.
  
  # Create Time object for current time.
  t = Time.now
  print t, "\n"
  
  # Check if it is daylight savings time.
  print t.dst?, "\n"
  
  # Check the weekday number
  # (0 == SUN, 1 == MON, 2 == TUE, etc.)
  print t.wday, "\n"
  
  # Find number seconds since Dec. 31, 1969, 18:00:00.
  print t.to_a, "\n"
  
  # Create time object for July 4, 1776, 12 noon:
  s = Time.new(1776, 7, 4, 12, 0, 0)
  s.print s, "\n"
  

• The following Ruby statements that add two Student objects to the students database, then checks the row count of the database:
  

  # Ssmple statements to use in your
  # test-student.rb file for Project 3.
  
  s1 = Student.new
  s1.first_name = "Sally"
  s1.major = "English"
  s1.year = 2
  s1.gpa = 3.87
  s1.save
  
  s2 = Student.new
  s2.first_name = "Edward"
  s2.major = "Art"
  s2.year = 3
  s2.gpa = 3.31
  s2.save
  
  print Student.count, "\n"
  
  print Student.first.first_name, "\n"