M359 - Conceptual Data Model - problems

Its been a frustrating week. In the course materials we are given a scenario for a University (well, what a surprise!) and a Conceptual Data Model that goes with it. The problem that I have had is that the Entity-Relationship Diagram shows an entity called Staff and another called Course (there are many other entities too). These two entities are linked by a one-to-many 'TelephoneTutoredBy' relationship with the 'one' being at the Course end and the 'many' at the Staff end. My reading of this is that each Course must have one, or more, staff members who do TelephoneTutoring for it. There is also a constraint that says that the staff member doing the telephone tutoring must also tutor at least one enrolment on that course. All pretty straight-forwards.

But the course text then went on to say that a staff member could ONLY telephone tutor on ONE course - and I couldn't see where this was stated in the Conceptual Data Model - there didn't appear to be any constraint anywhere saying that. I raised this as a question on FirstClass. Some responses said that the phrase "...staff member can only be a telephone tutor for a course if they are..." really meant "...a staff member can only tutor for ONE course if they are...". I don't buy the idea that 'a' really means 'one' - its ambiguous. If they had meant 'one' then they would have written 'one', surely. Another respondent suggested that it didn't matter and that I should just ignore the ambiguity (really!). After several days of pondering over this, there was a posting by the conference moderator who had written a guide to interpretting CDM's that went some way towards explaining the situation - but I still felt uneasy.

In the end I think the penny has finally dropped - the Entity-Relationship Diagram is only part of the model. Following the ERD is a list of 'Additional Constraints' that go on to clarify, or complement, the ERD. I think my problem was that I had glossed over the word 'Additional' and had interpretted it as being the textual representation of the diagram (ie; two facets of the same thing - they are not).

Having said that, I still don't see anywhere in the whole Conceptual Data Model where it says that staff can only TelephoneTutor on one course - but I am going to let it lie - the course text clearly make the assumption that it is.

Time to worry about the TMA!