Re: Why don't we have referencing subject/predicate/graph maps?

There are two constraints in this DB schema (note: changed the table 
name from "Enrolled" to "Enrollment" to make it more intuitive):
- Student.s_id = Enrollment.s_id
- Course.c_id = Enrollment.c_id

Since Enrollment is a many-to-many table, the primary key and hence the subject (assuming no duplicates) is: <s_id, c_id>.

We can express the two constraints easily as follows:

<#LinkMapStudentToCourse>
    rr:logicalTable [ rr:tableName "Enroll*ment*" ];
    rr:subjectMap <#*EnrollmentSubjectMap*>;
*    rr:predicateObjectMap [
        rr:predicateMap [ rr:predicate rr:involedStudent ];
        rr:objectMap [
            rr:parentTriplesMap <#StudentTriplesMap>;
            rr:join [ rr:child "s_id"; rr:parent "s_id" ];
        ]
    ];
*    rr:predicateObjectMap [
        rr:predicateMap [ rr:predicate rr:*involvedCourse* ];
        rr:objectMap [
            rr:parentTriplesMap <#CourseTriplesMap>;
            rr:join [ rr:child "c_id"; rr:parent "c_id" ];
        ]
    ].


This is a clean mapping between DB constraints (which are virtual cols 
whose values are primary key for the referenced table) and RDF predicates.
There is no need to try to create a constraint with the subject. 
(Subject is a multi-col primary key, not just the s_id column.)

Thanks,
- Souri/Seema.