Figure 7 ORM subschema for library membership.
By default, predicates are read left to right and top to bottom. A reversed reading direction is indicated
by a back arrow “<<”. The first role of the fact type Person was born on Date is optional. This means it is
optional whether we record a person’s birth date (even though in the real world each person has a birth
date). An ORM model reflects the universe of discourse (i.e. those aspects of the application world that we
wish to discuss, and the rules that we wish to enforce), so the model need not agree in every respect with
the real world. In this aspect, ORM differs from DEMO, where birthdate is mandatory simply because each
person in the real world has a birthdate.
The life-buoy symbol (combination of inclusive-or and exclusion symbols) denotes an exclusive-or
constraint: each Person was born on a Date or had alternative minimum age approval, but not both. Here the unary fact
type caters for the case where a person does not supply his/her birthdate, (e.g. he/she may not wish to
divulge it, or might not know it) but can have the minimum age requirement approved by authorized library
staff (e.g. visual inspection of a person who is obviously old).
Notice the use of hyphens in the fact types Year has minimum- member Age and Year has normal- loan Period.
This causes the hyphenated and any subsequent words before the following term for the object type to be
bound to that term for verbalization purposes. For example, the uniqueness constraint on the first of these
fact types verbalizes as “each Year has at most one minimum member Age” instead of “each Year has minimum member
at most one Age”.
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