Sessions

1
Sessions

• Carol Wolf
• Computer Science

2
HTTP Hypertext Transfer Protocol

• Protocol may refer to (from Wikipedia)
• Protocol (diplomacy), the etiquette of diplomacy
  and affairs of state
• Protocol (politics), a formal agreement between
  nation states
• Protocol, a.k.a. etiquette
• Clinical protocol, a.k.a. guideline (medical)
• Research methods
• Protocol (natural sciences)
• Clinical trial protocol
• Voice procedure a protocol for voice
  communications
• Protocol (object-oriented programming), a common
  means for unrelated objects to communicate with
  each other
• Communications protocol, in telecommunications
  and computer networking
• Protocol (band), British
• Protocol (film), a 1984 comedy film
• Protocol (album), the début album/EP from Simon
  Phillips

3
HTTP

• A single web transaction consists of a request,
  service and response.
• Once the transaction is finished, the server does
  not retain any memory of it.
• A standard solution is to store some information
  about the transaction in a cookie on the users
  computer.
• This is usually an id. It is important that no
  two users get assigned the same id.
• Web sites with heavy traffic can use GUIDs. A
  GUID is a Global Unique IDentifier. Generally, a
  GUID consists of a random number created using
  the time on the computer in nanoseconds and some
  feature of the server. In the past this was the
  network card MAC (Media Access Control) address.
  But after the Melissa worm used this to infect
  computers world-wide, Microsoft changed the GUID
  algorithm. The string, 3F2504E0 4f89 11D3 9A 0C
  03 05 E8 2C 33 01, is an example of a GUID. The
  numbers are in hexadecimal.
•  This id is then stored in the session on the
  server. When a user returns, the id is obtained
  from the cookie in order to find the appropriate
  user (and usually the users cart).

4
The Rails Session

• Web application frameworks all come with
  sessions.
• In Rails, the information in the session is
  stored in a hash.
• The size limit is 4K, so usually only an id is
  stored.
• And the id is most often a shopping cart id.
• The following example will be a student
  registration example.
• Given a list of courses, a student can choose the
  courses he/she wishes to take and collect them in
  a cart.
• When done the student would have to log into
  his/her university account.

5
The index page
6
Files generated

• Generate the courses table
• rails generate scaffold course numberstring
  namestring creditsinteger
• Generate the carts table. Carts are only needed
  for their ids.
• rails generate scaffold cart
• Generate the choices table
• rails generate scaffold choice cart_idinteger
  course_idinteger
• Create the tables with
• rake dbmigrate
• Add some courses to the courses table to use as
  examples.

7
The cart

• The cart id will be stored in the session and
  then used by the choices table.
• This is done in the application controller.
• class ApplicationController lt ActionControllerB
  ase
• protect_from_forgery
•  
• private
• def current_cart
• Cart.find(sessioncart_id) See if a cart
  already exists.
• rescue ActiveRecordRecordNotFound If not,
  create a new one.
• cart Cart.create
• sessioncart_id cart.id Store the cart
  id in the session hash.
• cart return the cart
• end
• end

8
The choices table

• The choices table is used to connect the student
  (via the cart) to the courses.
• Connections are made using belongs_to.
• class Choice lt ActiveRecordBase In the model
  folder
• belongs_to course
• belongs_to cart
•   The credits method is used to return the
  credits for the chosen course.
• def credits
• course.credits
• end
• end

9

• On course can be chosen by many students.
• All operations on the model (table) will be
  ordered by number.
• class Course lt ActiveRecordBase
• default_scope order gt 'number'
• has_many choices
•  
• validates number, name, credits, presence
  gt true
• validates credits, numericality gt
  greater_than_or_equal_to gt 1
• validates number, uniqueness gt true
• end

10
The Cart class

• class Cart lt ActiveRecordBase
• has_many choices, dependent gt destroy
•  
• def add_course(course_id)
• current_choice choices.where(course_id gt
  course_id).first
• If the course has not been chosen already,
  create a new choice.
• if current_choice nil
• current_choice Choice.new(course_id gt
  course_id)
• choices ltlt current_choice Append the new
  choice to choices.
• end
• current_choice return value
• end
•  
• def total_credits Used to add up the credits
  chosen so far.
• choices.to_a.sum list list.credits to_a
  makes an array of the choices.
• end
• end

11
The cart class

• The cart class connects to the choices table.
• When the cart is emptied, all dependent records
  will also be removed.
• The cart class is used to add a course to the
  choices table. This requires both the cart id
  (representing the student) and the course id.
• If the cart_id/course_id combination is not in
  the choices table, the new record is added. If
  it is, nothing is done. In either case, the
  record is returned.
• The total_credits method is used to keep track of
  the number of credits already in the cart.

12
View to change
13
Change the list_courses view

• lt _at_courses.each do course gt
• lttrgt
• lttdgtlt course.number gtlt/tdgt
• lttdgtlt course.name gtlt/tdgt
• lttdgtlt course.credits gtlt/tdgt
• lttdgtlt button_to "Add Course to Schedule",
• choices_path(course_id gt course.id) gt
•   lt/trgt
• lt end gt
• Add the button to the table. This sends the id
  of the chosen course to the choices controller.

14
Choices controller create method

• def create
• _at_cart current_cart found in application
  controller
• course Course.find(paramscourse_id)
• current_choice Choice.find_by_course_id(course
  .id)
•  
• if current_choice nil This is a new
  addition.
• _at_choice _at_cart.add_course(course.id)
• respond_to do format
• if _at_choice.save
• format.html redirect_to(_at_choice.cart,
• notice gt 'Course was successfully
  added.')
• else
• format.html render action gt "new"
• end
• end

15
Choices controller create method (more)

• else
• _at_choice current_choice
• respond_to do format
• format.html redirect_to(_at_choice.cart,
• notice gt 'Course was a duplicate and not
  added.')
• end
• end
• end

16
Choices controller create method

• First the method gets the current carts id from
  the session using the application controller.
• It then uses the id it received from the web page
  parameters to find the course in the choices
  table.
• If this course, the current_choice, has not yet
  been added to the table, it now is and a notice
  is flashed to the user.
• If it is already in the cart, no changes are made
  and a notice to this effect is flashed back.
• In either case, control is redirected to the show
  web page in the views/carts folder.

17
Show web page in views/carts folder

• ltp id"notice"gtlt notice gtlt/pgt
• lttablegt
• lt for item in _at_cart.choices gt
• lttrgt
• lttdgtlt item.course.number gt lt/tdgt
• lttdgtlt item.course.name gtlt/tdgt
• lttdgtlt item.course.credits gtlt/tdgt
• lt/trgt
• lt end gt
•   lttrgt
• lttd colspan"2"gtTotal Creditslt/tdgt
• lttdgtlt _at_cart.total_credits gtlt/tdgt
• lt/trgt
• lt/tablegt
•  

18
Show web page in views/carts folder

• Add a button to empty the cart (and start
  over).
• ltpgtlt button_to "Empty Cart", _at_cart,
• method gt delete, confirm gt 'Are you
  sure?' gtlt/pgt
•   Link back to the main page.
• ltpgtlt link_to 'Back', schedule_index_path
  gtlt/pgt
• The show method will show the cart and allow a
  student to empty it and start over, if desired.

19
Two versions of the show web page
20
Emptying the cart in views/carts

• Change one line in the delete method for the
  cart.
• def destroy
• _at_cart Cart.find(paramsid)
• _at_cart.destroy
•  
• respond_to do format
• format.html redirect_to(schedule_index_url)
  
• format.xml head ok
• end
• end
•