Courses

The SML spec describes an example application which keeps track of students and courses at a university. (Different examples in different parts of the spec describe slightly different concrete syntax, but see Appendix C for the fullest account.)

For each student, the system tracks the courses the student is enrolled in for the current term. The courses enrolled in are named by reference: in the examples of Appendix C, the u:EnrolledCourse element refers to a u:Course element in another XML document. We would like to be able to specify that every course-reference from the student's record refers to an actual course

For each approved course, the system documents the course code (department and course number), title, and prerequisite courses. The prerequisite courses are named by reference. We would like to be able to specify that every reference to a prerequisite from any course record refers to an actual course that no course is directly or indirectly a prerequisite for itself (if this occurs, it's an almost certain indication of corrupt data)

How are these constraints to be expressed? In principle, it appears to depend on how, in particular vocabulary, we recognize student records, course records, and the particular kind of reference involved in a constraint:

If we recognize a course record by means of the element's expanded name, then the constraint on student records can take the form The target of this reference must be an element with the expanded name {http://www.university.example.org/ns} Course.

This is what sml:targetElement does.

If we recognize a course record by means of the element's type, then the constraint on student records can take the form The target of this reference must be an element with the governing type {http://www.university.example.org/ns} CourseType.

This is what sml:targetType does.

However we recognize a course-prerequisite reference (as distinct from any other references which have a course as their target or a course as their source), we need so say The references of this set of reference must not form a cycle.

This is what sml:acyclic does.

Where do the constraints get imposed? If we recognize a student record's reference to a course, or a course's reference to a prerequisite course, by means of its expanded name (or more precisely by means of its being bound to a particular governing element declaration), then we want either to put the sml:targetElement or sml:targetType constraint on that element declaration (e.g. on {http://www.university.example.org/ns} EnrolledCourse or {http://www.university.example.org/ns} Prereq), or to put the constraint on a type which is used only for the one particular element. If we recognize the particular references by means of their governing type definitions, then we want to put the sml:targetElement or sml:targetType constraint on that type definition (e.g. on {http://www.university.example.org/ns} EnrolledCourseType or {http://www.university.example.org/ns} PrereqRefType).

Refinements and restrictions on courses

What happens when we wish to refine the schema?

If we introduce a distinction between graduate courses and undergraduate (upper and lower division) students and courses, we don't want to wish to lose the constraints.

We may also wish to refine the constraints. For example, we might wish to specify that: No undergraduate course may have any graduate course among its prerequisites. No lower-division undergraduates may have any graduate courses among their enrolled courses.

Two aspects need to be considered: the target and the source of the reference. If the target is identified by type, any refinement of the target will continue to satisfy the constraint. If the target is identified by element declaration, do other elements in that element's substitution group satisfy the constraint? If the source is identified by type, any refinement of the source or its containing type will continue to impose the constraint. If the source is identified by element declaration, then we need to ensure that refinements of the containing type satisfy the constraint.

Hosting dependencies

Imagine an SML model for an IT center, in which there are documents describing ...

[Description, examples]

We would like to enforce the following constraints: The depends-on relation must not contain cycles. [more ...]