Designing a relationship with three entities where two entities have no relation

Question

Trying to implement an ER model where I have entities:teacher,student,papers and relationship: publishes,advises. Both teacher and student can publish a paper but only a teacher can advise a paper. Should I duplicate publishes relationship for both student and teacher or can I make it look like a three-way relationship with having no relationship between teacher and student?

Answer 1

It sounds like you could model it like:

student(student_id, name, etc)
teacher(teacher_id, name, etc)
paper(paper_id, title, text, etc)
contributor(contributor_id, paper_id, contribution_type, contributor_type)

Where contribution type is an enum(publisher,adviser) and similarly contributor type is an enum(teacher,student)... or booleans is_publisher, is_adviser.

The drawback is that this doesn't permit foreign keys from contributor to student/teacher, and you don't have a rigid constraint from advisers to teachers. A table adviser(teacher_id, paper_id) allows a constraint on the advisers, but still doesn't allow constraints or foreign keys on student ids.

Another options might be to break it up as:

teacher_contribution(teacher_id, paper_id, is_adviser)
student_contribution(student_id, paper_id)

which would allow completely constraining the database to the intended model, but could be more difficult to query in some situations.

Any are acceptable. It depends to some extent on your particular application and how you intend to query the data.