To Lecture Notes

IT 232 -- 10/12/15

Review Exercises

1. What is the value of @c after these controller statements?
   

   a = [2, 3, 5, 8, 9, 12]
   b = [3, 5, 6, 9, 11]
   @c = a - b
   
   Ans:
   [2, 6, 12]
   

2. What is the spaceship operator?
   Ans: It is an operator that looks like this: <=>. It performs a three-value comparison for objects. If n > m, n <=> m returns 1; if n < m, n <=> m returns -1; if n == m, n <=> m returns 0. If the <=> operator is defined in the model, it can be used to tell the Ruby sort method how to sort an array of objects.
3. What are the values of @e and @f after these controller statements?
   

   n = 3
   m = 8
   @e = "%d %d %d\n" % 
     [n <=> m, m <=> n, m <=> m]
   
   s = "orange"
   t = "apple"
   @f = "%d %d %d\n" % 
     [s <=> t, t <=> s, t <=> t]
     
   Ans:
   -1 1 0
   1 -1 0  
   

4. Try out this ERB code for pluralizing a noun:
   

   <p>User has <%= pluralize(@n, "photo") %>.</p>
   

   Set the value of @n in the controller.
5. What does self mean?
   Ans: self refers to the current object in the model code.
6. Write a Ruby program that reads the Edgar Allen Poe poem poem.txt and display it in a view.
   Ans: See the Poem Example.

Project 2

• Look at selected Project 2 submissions.

Many-to-many Relationships

• The three methods for implementing a many-to-many relationship
  
  1. Set up a one-to-many relationship between the models Course and Enrollment; also set up a one-to-many relationship between Student and Enrollment. Example: See the courses, students, and enrollments tables.
  2. Instead of using the Enrollment model, use a courses_students table and the has_and_belongs_to_many statement to connect the Course and Student models.  Note that for the courses_students table, courses and students must be in alphabetic order.
  3. Use the Enrollment model and use these model statements to connect the Course and Student models:
     

     # in the model file app/models/course.rb:
     has_many :enrollments
     has_many :students, through: enrollments
     
     # in the model file app/models/student.rb:
     has_many :enrollments
     has_many :courses, through: enrollments
     

• See the School2 and School3 Examples.

Sorting an Array of Objects

• Sometimes you will need to sort an array that is not an ActiveRecord relation.
• For an ordinary array of objects (not an ActiveRecord relation) to be sortable, a spaceship operator ( <=> ) is defined so that the object's class can compare its objects.
• Last time we modified the School1 Example to show the unenrolled courses in radio buttons to allow a student to enroll in a class. How can we show the courses in sorted order? Ans:
  
  1. Add this definition of <=> to the Course model file app/models/course.rb:
     

     def <=>(other_course)
       self.course_num <=> other_course.course_num
     end
     

  2. In the Student show view at app/views/students/show.html.erb, replace the line
     

      <% @student.unenrolled_courses.each do |course| %>
     

     with
     

      <% @student.unenrolled_courses.sort.each do |course| %>
     

• Defining <=> and using the sort method to sort an array is restrictive because the definition of <=> is hardcoded into the model.
• A more powerful way to select and sort objects from a model is to use ActiveRecord statements. These are discussed later.