Example system: Enrollment

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In this section, you will learn the basic structure of information systems according to Ampersand. By studying a simple system, you will learn how Ampersand represents such systems.
We will study an information system called "Enrollment". The purpose of that system is to enroll students for modules. Students can enroll (or be enrolled) for any module that is part of the course they take.
Try it out on an Ampersand implementation. Copy this example code, make a new script on an Ampersand system, compile it, and generate a prototype implementation from it. Run the prototype and then click on the icon "Overview" in the top-left of the screen. The application will start. Browse through the data and change things. Find out which courses, students, and modules there are and try to see what happens if you add or remove information from the system. You can use this assignment as a guide:
Assignment
Who are the three students and what are the courses they take?
Is there a module HRM?
Add, in the tab Course, the module HRM to the Management course. The system sees that the subject is unknown, and provides a green plus-sign to add it.
Each change will be saved automatically.
Note that this change is also visible in the tab Modules.
Now enroll student John for the module Business Rules. You can do this either in the tab Students or the tab Modules. We will later see why this is the case.
Now enroll student Peter for the module Business Rules.
This will trigger a message because this student does not take the course Business IT.
Click on the message to see details.
Look-up in the code below where this message is defined. It is the line that starts with RULE. You don't have to understand the syntax at this point.
Click cancel.
Use the trash can icon to remove the course Business IT fromSusan in the tab Students. 
This also will trigger an error message because each student can only enroll for a module in courses he has registered in. Get rid of the message by going to the "List all interfaces" in the menu bar and navigate back to the overview.
Look up in the code the definition of the RELATION takes. The keyword [TOT] is responsible for this message.
Note the four icons on the top-right of the screen and click on the second icon (nine dots).
Click on 'Reinstall database'
The Installer screen comes up. Click on the big red button and wait for it to turn green.
Click on 'overview' and the initial data is back.
Have a look at the code below, to find where the initial data is defined.
Have a look at the code-part at the end called INTERFACE and compare it with the screen. Note which parts you recognize. The syntax of the code is discussed in the documentation.
This information system was built by the following code:
CONTEXT Enrollment IN ENGLISH
PURPOSE CONTEXT Enrollment
{+ A complete course consists of several modules.
Students of a course can enroll for any module that is part of the course.
+}
​
PATTERN Courses
-- The concepts
CONCEPT Student "Someone who wants to study at this institute"
PURPOSE CONCEPT Student 
{+We have to know what person studies at this institute, so the system needs to keep track of them.+}
CONCEPT Course "A complete course that prepares for a diploma"
PURPOSE CONCEPT Course 
{+We have to know what courses there are, so the system needs to keep track of them.+}
CONCEPT Module "An educational entity with a single exam"
PURPOSE CONCEPT Module 
{+We have to know what modules exist, so the system needs to keep track of them.+}
​
-- The relations and the initial population
RELATION takes [Student*Course][TOT]
MEANING "A student takes a complete course"
​
POPULATION takes CONTAINS
[ ("Peter", "Management")
; ("Susan", "Business IT")
; ("John", "Business IT")
]
​
RELATION isPartOf[Module*Course]
MEANING "A module part of a complete course"
​
POPULATION isPartOf[Module*Course] CONTAINS
[ ("Finance", "Management")
; ("Business Rules", "Business IT")
; ("Business Analytics", "Business IT")
; ("IT-Governance", "Business IT")
; ("IT-Governance", "Management")
]
​
RELATION isEnrolledFor [Student*Module]
MEANING "Students enroll for each module in the course separately"
​
-- The one rule in this model
RULE ModuleEnrollment: isEnrolledFor |- takes;isPartOf~
MEANING "A student can enroll for any module that is part of the course the student takes"
MESSAGE "Attempt to enroll student(s) for a module that is not part of the student's course."
VIOLATION (TXT "Student ", SRC I, TXT " enrolled for the module ", TGT I, TXT " which is not part of the course ", SRC I[Student];takes)
ENDPATTERN
​
INTERFACE Overview : "_SESSION"                  cRud
BOX <TABS>
     [ Students : V[SESSION*Student]             cRuD
       BOX <TABLE>
                [ "Student" : I[Student]         cRud
                , "Enrolled for" : isEnrolledFor cRUD
                , "Course" : takes CRUD
                ]
     , Course : V[SESSION*Course]                cRuD
       BOX <TABLE>
                [ "Course" : I                   cRud
                , "Modules" : isPartOf~          CRUD
                ]
     , Modules : V[SESSION*Module]               cRud
       BOX <TABLE>
                [ "Modules" : I                  cRuD
                , "Course" : isPartOf            cRUd
                , "Students" : isEnrolledFor~    CRUD
                ]
     ]
​
ENDCONTEXT
​
What have you learned?
After finishing your assignment, you have learned:
to recognize details in the source code of your information system and relate them to the information system "Enrollment" that you have been playing with;
that a rule of the business, such as "A student can only enroll for a module that is in the course the student takes" can be formalized in Ampersand.
that such a business rule can be used to constrain data in a database.
Want to learn more?
1.How can I create my own information system in RAP4? Go to Your tool: RAP4.
2.What is the conceptual model behind an Ampersand model? Go to Conceptual Model: Enrollment.​
3.How can I upload bulk data from spreadsheets into my application?
Tutorial
Conceptual Model: Enrollment
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Assignment
What have you learned?
Want to learn more?