To Lecture Notes

IT 231 -- 9/20/12

Review Questions

1. What is wrong?
   

   rails g new controller index p 1
   

   Ans: The word new should not be here and there should not be a space between p and 1 because actions cannot contain spaces. The scaffold software will think that the actions are index, p, and 1, but 1 is not a legal name for an action. The controller name is also missing between "controller" and "index".
    
2. What are the names of the database tables generated by the following models in a scaffold-based application?
   

   Student   GroceryItem   Box   Story   Person
   
   Ans: students  grocery_item  boxes  stories  people
   

3. What is wrong?
   

   rails new scaffold generate People name:int gender:Text class:String
   

   Ans: The word new should be omitted; the words scaffold and generate should be reversed. Optionally, you can abbreviate generate to g. The model name People should be singular (Person), int should be string, text should be string and lower case because the gender is only one letter. Even if you want to use an integer, spell out the datatype as "integer", not "int". class is a reserved word, so use another word like course instead, string should be lower case.
    
4. What are the names of the following items created by a Rails scaffold-based project using the model GroceryItem?
   
   1. resources  i Ans: grocery_items
   2. database table   Ans: grocery_items
   3. controller file   grocery_item_controller
   4. folder containing views   grocery_item
   5. URL of index view   http://localhost:3000/grocery_items
   
   
5. What does
   

   rake db:migrate
   

   do?
    
   Ans: It runs the latest migration file, which may create the database table, or modify the items in the database table.
    
6. What is the name and location of the underlying database for a Rails project generated using a scaffold?
   Ans: db/development.sqlite3
    
7. Which views does the Rails scaffold software create?
   Ans: index, show, new, edit.
    
8. Explain the difference between 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.
    
9. What is the filename containg the code that the following render command inserts?
   

   <%= render 'page' %> 
   

   Ans: _page.html.erb
    
10. 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 with
    

    rails dbconsole
    

11. How do you determine the controller and action to use to display A Rails view by looking at the Rails project?
    Ans: Look in app/views. You will find a folder named layouts and another folder. The other folder is the controller name. Go into this folder the .html.erb files in this folder correspond to the names of the views.
     
12. How does a view know (a) if it is supposed to use a layout page and (b) which layout page to use?
    Ans: (a) A view indicates that it wants to use a layout page if there is either a file named <controller_name>.html.erb or application.html.erb in app/views/layouts. (b) If both files are present, the one with the controller name is used. Of course, the layout page must contain the ERB statement
    

    <%= yield %> 
    

    for the layout page to work.
     
13. Create a scaffold-based project named Login with fields username, password, and expiration_date. Then do the following:
     
    
    1. Change the years displayed in the year dropdown box for specifying :expiration_date to start at 2012 and end at 2016.
       Ans: This answer is shown for the MovieReviewSite Example. Change Line 24 of _form.html.erb from
       

       <%= f.date_select :movie_release_date %>
       

       to

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

    
     
    2. Write validations to do the following:
        
       
       1. The field :owner must be nonempty. Ans:
          

          validates :username, :presence => true
          

          
           
       2. The field :password must have a minimum length of 8 and maximum length of 20.Ans:
          

          validates :password, :length => { :in => 8..20 }
          

Helper Methods for URLs Generated by Scaffold Software

• Here are some Rails helper methods generated by the Rails scaffold software from the students 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 %>
  

Project 2

• Look at the Project 2 Description.

Moving Web Pages over the Internet

• In the early days of computer networking (pre 1969), connections between computers were hardwired, and the software to implement the connection was custom tailored to the operating systems of the connected computers.
   
• As the ARPANET (predecessor of the modern Internet) was being built, it became clear that connecting a large number of computers together in one large network required a well designed network architecture.
   
• Gradually the Open Systems Institute (OSI) Seven Layer Model was developed, starting in 1977.
   
• This Seven Layer Model was used to organize network software and allow a wide variety of computer network systems to be connected into a single network.
   
• Of special interest is the HTTP software, which sits at the very top of the Seven Layer Model in the Application layer.
   
• Modern telecommunications systems send data in relatively small packets over the best available transmission line. Then the packets are collected and reassembled to present to the application software on the destination machine.
   
• See this document for a taste of HTTP in the early days:
   
  
  Using HTTP for Client-to-server Communication

Running WEBrick on a Different Port

• To start Webrick on a different port:
  

  rails server --port=5000
  

• Then use this URL:
  

  http://localhost:5000/grocery_items
  

• The default port for HTTP is 80. If WEBrick is started as
  

  rails server --port=80
  

  then the page can be viewed without specifying a port number:
  

  http://localhost/grocery_items
  

• Two Rails servers running at the same time on the same machine must run on different ports.
   
• Here is a list of some other well-known ports.
   
• Well-known ports are in the range from 0 to 1023.
   
• Know the port number usually associated with HTTP, which is 80.

HTTP GET vs. POST

• Submit information via HTTP GET:
   
  
  submit-with-get.htm
  
  
• The page information is submitted back to the server in the URL; for example:
  

  display1.htm?txtName=Alice&txtGender=F&txtAge=29
  

• Submit information via HTTP POST:
   
  
  submit-with-post.htm
  
  
• The page information is not submitted back to the server in the URL; it is submitted in the page head.

Representational State Transfer

• Representational state transfer (REST) means using the HTTP commands GET, POST, PUT, and DELETE operations to implement the CRUD operations:
   
• REST also assumes
   
  
  1. use of a client-server architecture
      
  2. stateless communication, which means that the neither the views nor the controller remembers the contents of previously submitted pages.
  
  
• Defining the following route in config/routes.rb creates the entire set of CRUD operations:
  

  map.resources :items
  

• Some details of REST operations, when the resources are defined as :items.