To Lecture Notes

IT 231 -- 11/8/12

Review Questions

1. What is state for a website?
   Ans: The state of a web session is information that is valid for multiple request and response exchanges between the client and the server. The state might include the logged-in status of a client, the number of times a page has been visited, customer information a logged in client, etc.
    
2. How do the session variables help keep track of state?
   Ans: State information can be stored as a session variables, as shown in the following question.
    
3. How do you set the session variable with key :logged_in to true.
   

   session[:logged_in] = true
   

   
    
4. What do you think is printed?
   

   a = [32, 54, 33]
   print "***#{a[3]}***\n"
   
   Ans: 
   ******
   a[3] returns nil because 3 is not a legal index (the legal indices are
   0, 1, and 2).  When nil is printed, nothing happens, so there is nothing
   between the first three stars and the second three.
   

5. What is the value of a session variable that has not been set.
   Ans: nil
    
6. Design a scaffold-based website that implements a shopping cart. Use these models: Customer, CatalogItem, ShoppingCart, LineItem

Migrations

• Create a Rails model named Student with the fields name:string, student_id:integer, gpa:float. Then do the following:
   
  
  1. Add a new field gender:string.
     

     rails g migration AddGenderToStudents gender:string
     

  2. Remove the field gpa.

  rails g migration RemoveGpaFromStudents gpa:float
  

  3. Look at the migration class files generated in the db/migrations folder.
      
  4. A migration class usually has an up method that is executed when the migration is executed with rake db:migrate; it also generally has a down method that is executed with rake db:rollback.
      
  5. The migration class file also has a number that specifies the order in which migrations are executed.

Error Handling for Validations

• Create a Rails project named Store with a model named Order having fields cust_name, phone, item_id, and price.
   
• Add these validations to the Order model:
  

  validates :cust_name, :presence => true
  validates :price, :numericality => true
  

• Add a controller named ManageOrders with two views: (1) new_order to enter and save a new order in the database, and (2) display_orders to show all the orders currently in the database.
   
• Look at this error handling code modified from the app/views/students/_form.html.erb file of a Rails scaffold project:
  

  <% if @student.errors.any? %>
     <h2>These errors prohibited this student from being saved.</h2>
     <ul> 
        <% @student.errors.full_messages.each do |msg| %>
           <li><%= msg %></li>
        <% end %>
     </ul>
  <% end %>
  

• Use this error handling code to show errors on the display_orders view when the validations are violated in this Store project.
   
• The code is given in the Store Example (Example 32.5).