Cascading List of Values with many to many relationship - plsql

Question

I am developing an application which tracks class attendance of students in a school, in Apex.
I want to create a page with three level cascading select lists, so the teacher can first select the Semester, then the Subject and then the specific Class of that Subject, so the application returns the Students who are enrolled in that Class.
My problem is that these three tables have a many-to-many relationship between them, so I use extra tables with their keys.
Every Semester has many Subjects and a Subject can be taught in many Semesters.
Every Subject has many classes in every Semester.
The students must enroll in a subject every semester and then the teacher can assign them to a class.
The tables look something like this:
create table semester(
id number not null,
name varchar2(20) not null,
primary key(id)
);
create table subject(
id number not null,
subject_name varchar2(50) not null,
primary key(id)
);
create table student(
id number not null,
name varchar2(20),
primary key(id)
);
create table semester_subject(
id number not null,
semester_id number not null,
subject_id number not null,
primary key(id),
foreign key(semester_id) references semester(id),
foreign key(subject_id) references subject(id),
constraint unique sem_sub_uq unique(semester_id, subject_id)
);
create table class(
id number not null,
name number not null,
semester_subject_id number not null,
primary key(id),
foreign key(semester_subject_id) references semester_subject(id)
);
create table class_enrollment(
id number not null,
student_id number not null,
semester_subject_id number not null,
class_id number,
primary_key(id),
foreign key(student_id) references student(id),
foreign key(semester_subject_id) references semester_subject(id),
foreign key(class_id) references class(id)
);
The list of value for the Semester select list looks like this:
select name, id
from semester
order by 1;
The the subject select list should include the names of all the Subjects available in the semester selected above, but I can't figure the query or even if it's possible. What I have right now:
select s.name, s.id
from subject s, semester_subject ss
where ss.semester_id = :PX_SEMESTER //value from above select list
and ss.subject_id = s.id;
But you can't have two tables in a LoV and the query is probably wrong anyway...
I didn't even begin to think about what the query for the class would look like.
I appreciate any help or if you can point me in the right direction so I can figure it out myself.

Answer 1

Developing an Apex Input Form Using Item-Parametrized Lists of Values (LOVs)
Your initial schema design looks good. One recommendation once you've developed and tested your solution on a smaller scale, append to the ID (primary key) columns a trigger that can auto-populate its values through a sequence. You could also skip the trigger and just reference the sequence in your sql insert DML commands. It just makes things simpler. Creating tables in the APEX environment with their built-in wizards offer the opportunity to make an "auto-incrementing" key column.
There is also an additional column added to the SEMESTER table called SORT_KEY. This helps when you are storing string typed values which have logical sorting sequences that aren't exactly alphanumeric in nature.
Setting Up The Test Data Values
Here is the test data I generated to demonstrate the cascading list of values design that will work with the example.
Making Dynamic List of Value Queries
The next step is to make the first three inter-dependent List of Values definitions. As you have discovered, you can reference page parameters in your LOVs which may come from a variety of sources. In this case, the choice selection from our LOVs will be assigned to Apex Page Items.
I also thought only one table could be referenced in a single LOV query. This is incorrect. The page documentation suggests that it is the SQL query syntax that is the limiting factor. The following LOV queries reference more than one table, and they work:
-- SEMESTER LOV Query
-- name: CHOOSE_SEMESTER
select a.name d, a.id r
from semester a
where a.id in (
select b.semester_id
from semester_subject b
where b.subject_id = nvl(:P5_SUBJECT, b.subject_id))
order by a.sort_id
-- SUBJECT LOV Query
-- name: CHOOSE_SUBJECT
select a.subject_name d, a.id r
from subject a
where a.id in (
select b.subject_id
from semester_subject b
where b.semester_id = nvl(:P5_SEMESTER, b.semester_id))
order by 1
-- CLASS LOV Query
-- name: CHOOSE_CLASS
select a.name d, a.id r
from class a, semester_subject b
where a.semester_subject_id = b.id
and b.subject_id = :P5_SUBJECT
and b.semester_id = :P5_SEMESTER
order by 1
Some design notes to consider:
Don't mind the P5_ITEM notation. The page in my sample app happened to be on "page 5" and so the convention goes.
I chose to assign a name for each LOV query as a hint. Don't just embed the query in an item. Add some breathing room for yourself as a developer by making the LOV a portable object that can be referenced elsewhere if needed.
MAKE a named LOV for each query through the SHARED OBJECTS menu option of your application designer.
The extra operator involving the NVL command, as in nvl(:P5_SUBJECT, b.subject_id) for the CHOOSE_SEMESTER LOV is an expression mirrored on the CHOOSE_SUBJECT query as well. If the default value of P5_SUBJECT and P5_SEMESTER are null when entering the page, how does that assist with the handling of the cascading relationships?
The table SEMESTER_SUBJECT represents a key relationship. Why is a LOV for this table not needed?
APEX Application Form Design Using Cascading LOVs
Setting up the a page for testing the schema design and LOV queries requires the creation of three page items:
Each page item should be defined as a SELECT LIST leave all the defaults initially until you understand how the basic design works. Each select list item should be associated with their corresponding LOV, such as:
The key design twist is the Select List made for the CHOOSE_CLASS LOV, which represents a cascading dependency on more than one data source.
We will use the "Cascading Parent" option so that this item will wait until both CHOOSE_SEMESTER and CHOOSE_SUBJECT are selected. It will also refresh if either of the two are changed.
YES! The cascading parent item can consist of multiple page items/elements. They just have to be declared in a comma separated list.
From the online help info, this is a general introduction to how cascading LOVs can be used in APEX designs:
From Oracle Apex Help Docs: A cascading LOV means that the current item's list of values should be refreshed if the value of another item on this page gets changed.
Specify a comma separated list of page items to be used to trigger the refresh. You can then use those page items in the where clause of your "List of Values" SQL statement.
Demonstration of APEX Application Items with Cascading LOVs
These examples are based on the sample data given at the beginning of this solution. The path of the chosen example case is:
SEMESTER: SPRING 2014 + SUBJECT: PHYS ED + Verify Valid Course Options:
Fitness for Life
General Flexibility
Presidential Fitness Challenge
Running for Fun
Volleyball Basics
The choice from above will be assigned to page item P5_CLASS.
Selection Choices for P5_SEMESTER:
Selection Choices for P5_SUBJECT:
Selection Choices for P5_CLASS:
Closing Remarks and Discussion
Some closing thoughts that occurred to me while working with this design project:
About the Primary Keys: The notion of a generic, ID named column for a primary key was a good design choice. While APEX can handle composite business keys, it gets clumsy and difficult to work around.
One thing that made the schema design challenging to work with was that the notion of "id" transformed in the other tables that referenced it. (Such as the ID column in the SEMESTER table became SEMESTER_ID in the SEMESTER_SUBJECT table. Just keep an eye on these name changes with larger queries. At times I actually lost track exactly what ID I was working with.
A Word for Sanity: In the likely event you decide to assign ID values through a database sequence object, the default is usually to begin at one. If you have several different tables in your schema with the same column name: ID and some associating tables such as CLASS_ENROLLMENT which connects the values of one primary key ID and three additional foreign key ID's, it may get difficult to discern where the data values are coming from.
Consider offsetting your sequences or arbitrarily choosing different increments and starting values. If you're mainly pushing ID's around in your queries, if two different ID sets are separated by two or three orders of magnitude, it will be easy to know if you've pulled the right data values.
Are There MORE Cascading Relationships? If a "parent" item relationship indicates a dependency that makes a page item LOV wait or change depending on the value of another, could there be another cascading relationship to define? In the case of CHOOSE_SEMESTER and CHOOSE_SUBJECT is it possible? Is it necessary?
I was able to figure out how to make these two items hold an optional cascading dependency, but it required setting up another outside page item reference. (If it isn't optional, you get stuck in a closed loop as soon as one of the two values changes.) Fancy, but not really necessary to solve the problem at hand.
What's Left to Do? I left out some additional tasks for you to continue with, such as managing the DML into the ENROLLMENT table after selecting a valid STUDENT.
Overall, you've got a workable schema design. There is a way to represent the data relationships through an APEX application design pattern. Happy coding, it looks like a challenging project!