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Figure 2: An ORM metaschema for IE
For each association, or fact type, a sample fact table may be added to help validate
the constraints. Each column in a fact table is associated with one role. The arrow-tipped
bars are internal uniqueness constraints , indicating which roles or role combinations must
have unique entries. A black dot on a role connector indicates the role is mandatory for
its object type. For example, the uniqueness and mandatory constraints on the association
Attribute has AttributeName in Figure 1 verbalizes respectively as: each Attribute has at
most one AttributeName ; each Attribute has at least one AttributeName . ORM schemas
may be represented in diagrammatic or textual form, and tools such as Microsoft Visio for
Enterprise Architects provide automatic transformation between the two representations
(Halpin et al., 2003).
The metaschema in Figure 2 assumes a closed world approach for the unary predicates
(e.g., if an attribute is not recorded to be mandatory, then it is known to be optional). It also
assumes that primary identifi er attributes must be mandatory—this is captured by the subset
constraint (circled “⊆”), which indicates that the population of the lower role must be a
subset of the upper role (i.e., if an attribute is a primary identifi er component then it must
be mandatory). The external uniqueness constraint (circled “u”) indicates that an attribute
may be identifi ed by combining its unqualifi ed name with its entity type.
For convenience, roles (association ends) and associations are identifi ed by numbers
(whose display is normally suppressed). Roles could also be identifi ed by their position
within a standard ordering of an association. Association names are catered for by attaching
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