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IT 231 -- 10/25/12

Review Questions

1. Write a Rails view that displays 1000 lines on a page like this:

   Line 1

   Line 2

   ...

   Line 999

   Line 1000

   Use the Range 1..1000 with the each method.

   # Ans: here is the view code:
   
   <Display 1000 Lines>
   
   <% (1..1000).each do |i| %>
   <p>Line <%= i %></p>
   <% end %>
   



2. Explain the difference between FormTagHelper and FormHelper methods.
    
   Ans: FormTagHelper methods are used to implements HTML forms and controls that are not connected to a database. FormHelper method work closely with the database to move data from the form to a database and vice versa. FormTagHelper method names all end with _tag. FormHelper method names do not end with _tag.
    
3. Find the mistakes in this view that uses FormHelper methods:
   

   <% form_for @cust, "display" |form|
   <p>Prefix: <% radio-button :prefix, 'Mr.', true %>
   <p>Prefix: <% radio-button :prefix, 'Ms.' %>
   <p>First Name: <% text-field :first_name, ' ' %>
   <% end %>
   
   <!-- Ans: Here is the corrected version: -->
   <%= form_for @cust, :url => { :action => 'display' } do |f| %>
   <p>Prefix: Mr. <%= f.radio_button :prefix, 'Mr.', :checked => true %>
                  <%= f.radio_button :prefix, 'Ms.' %>
   <p>First Name: <%= f.text_field :first_name %>
   <p><%= f.submit 'Submit Name' %>
   <% end %>
   

4. Find the mistakes in this controller code:
   

   @cust = new Customer[param(:cust)]
   
   # Ans: The corrected line is
   @cust = Customer.new(params[:customer])
   

5. Find the mistakes in this view code:
   

   <%= Customer.all.each |cust| %>
   <%= table %>
   <tr> <th>Name<th> <th>Customer Id</th> </tr> 
   <tr> <td>c.name<td> <td>c.cust_id</th> </tr> 
   <%= /table %>
   <% end %>
   
   <-- Ans: Here is the corrected output: -->
   <table>
   <tr> <th>Name<th> <th>Customer Id</th> </tr> 
   <% Customer.all.each do |c| %>
   <tr> <td>c.name<td> <td>c.cust_id</td> </tr> 
   <% end %>
   </table>
   

6. Create a Guestbook project using FormHelper methods. The model should be named Comment with fields name and message. The views for the Controller Comments should be new and display_all. Include the created_at field on the display_all view. Recall that the created_at field is automatically created when you create the model or scaffold.
   Ans: See Example 30 (GuestBook Example).

The Array Method collect

• The Array method collect is used to build a new array out of an existing array. It collects all the outputs from the code block and collects them all together go build a new array.
   
• Examples:   What is the output of each of these Ruby code fragments:

  a = [2, 6, 4, 5, 1]
  
  # Example 1
  b = a.collect do |x|
     2 * x
  end
  print b, "\n"
  
  Ans: [4, 12, 8, 10, 1]
  
  # Example 2
  b = (0..5).to_a.collect do |x|
     2 * x + 1
  end
  print b, "\n"
  
  Ans: [1, 3, 5, 7, 9, 11]
  
  # Example 3
  b = (1..5).to_a.collect do |x|
     [x, x**2]
  end
  print b, "\n"
  
  Ans: [[1, 1], [2, 4], [3, 9], [4, 16], [5, 25]]
  

One-to-many Relationships

• A primary key is a key that uniquely identifies a row in a database table.
   
• Rails always uses the column id to identify a row. This column is automatically created when the model is generated. id should not be specified in a model statement.
   
• A foreign key is a column in one table that refers to the primary key id in another table (which is always id in Rails).

• Foreign keys are used in relational databases to link rows in related tables.
   
• In a one-many relationship the foreign key column can have many rows that refer to the same primary key.
   
• Examples:
   
  
  One student takes many classes.
   
  One primary care doctor has many patients.
   
  One airplane has many passengers.
   
  One hotel has many guests.
   
  One advisor has many advisees.
   
  One house has many rooms.
   
  One play has many acts.
   
  One playlist has many songs.
   
  One student has many awards (in high school).
  
  
• In each case, the model listed first is the primary model. The model listed second is the dependent model.
   
• In each case, many means an integer greater than or equal to zero.
   
• Here are the steps to setting up a one-to-many relationship:
   
  
  1. Use Rails create model statements to create the two models. The "one" model is the primary model and the "many" model is the secondary model. The secondary model must have a field named <primary>_id, where you replace <primary> with the name of the primary model. Assume that the primary model is Flight and the secondary model is Passenger. Then the Passenger table must have the field named flight_id.
      
  2. Add this line to the model file (in apps/models/flight.rb) for the primary model:
     

     has_many :flights
     

  3. Add this line to the model file (in apps/models/passenger.rb) for the secondary model:
     

     belongs_to :passenger
     

• What does this one-many relation between the models Flight and Passenger do for you?
   
  
  1. It creates a Flight method passengers, which returns the array of all passengers belonging to that Flight object.
      
  2. It creates a Passenger method flight, which returns the Flight object to which it belongs.
  
  
• Look at the One-many Relationships Tutorial. We will complete this tutorial on Tuesday, October 30.
   
• Practice Problem:   Create a Rails scaffold project with two models Patient and Doctor. Use this seed file code to get the field names and to populate the database tables. Then add a new controller named DocInfo with a view display that
   
  
  1. shows a table with the Patient names and his or her doctor's name and phone number,
      
  2. shows an unordered list with the Doctor names with an unordered list of Patient name and age for each doctor.

Final Project

• Discuss the Final Website Project