ORM Glossary

This glossary lists key terms and symbols used in Object-Role Modeling (ORM), and briefly explains their meaning. A concise explanation of other technical terms may be found in the chapter summaries. Further details on technical terms may be accessed by using the Index.

Alethic constraint:

Constraint that holds necessarily for all states of the model.

Arity:

Number of roles in a relationship (unary = 1, binary = 2, ternary = 3, etc.).

Asserted fact:

Fact that is simply asserted, rather than being derived from others; also called a primitive fact or base fact.

Asserted subtype:

Subtype that is simply asserted (not defined by a subtype definition).

Association:

Relationship type, usually involving at least two roles.

Atomic fact:

Either an elementary fact or an existential fact.

Base fact:

Fact that is primitive (not derived from others). Also called an asserted fact.

Compound fact type:

Fact type that is equivalent to a conjunction of smaller fact types

Conceptual schema:

Conceptual model of the UoD structure; design that specifies what states and transitions are possible; declaration of fact types, constraints, and derivation rules

Conceptual schema design procedure (CSDP):

0 Divide the UoD into manageable sub-sections

1 Transform familiar examples into elementary facts, and apply quality checks

2 Draw the fact types, and apply a population check

3 Check for entity types that should be combined, and note arithmetic derivations

4 Add uniqueness constraints, and check arity of fact types

5 Add mandatory role constraints, and check for logical derivations

6 Add value, set comparison (subset, equality, exclusion) and subtype constraints

7 Add other constraints and perform final checks

8 Integrate the subschemas into a global conceptual schema

Constraint:

Restriction on possible or permissible states (static constraint) or transitions (dynamic constraint).

Compositely identified object type:

Either a coreferenced or a nested object type.

Coreferenced object:

Object that is identified by means of two or more reference types in combination; hence its identification scheme involves an external uniqueness constraint.

Database:

Variable set of related fact instances.

Deontic rule:

An obligation, i.e. a rule that ought to be obeyed (but possibly may be violated).

Derivation rule:

Rule that declares how one fact type may be derived from others.

Derived fact:

Fact that is derived from other fact types using a derivation rule

Derived subtype:

Subtype that is derived from other object types using a subtype definition.

Elementary fact:

Assertion that an object has a property, or that one or more objects participate in a relationship, where the fact cannot be split into simpler facts with the same object types without information loss. Application of an atomic predicate to a sequence of objects.

Entity:

Object that is referenced by relating it to other objects (e.g., the Country that has CountryCode ‘AU’); not a value; typically, an entity may undergo changes over time; an entity is either atomic or nested (i.e. an objectified relationship); at the top level, entities are partitioned into primitive entity types, from which subtypes may be defined.

Existential fact:

Assertion that an object exists (e.g., there is a Country that has CountryCode ‘AU’); also called a reference.

Fact:

Proposition that is taken to be true by the relevant business community, where the proposition is elementary or existential (rather than being a constraint or derivation rule).

Fact role:

Role in an elementary fact type.

Fact type:

Kind of fact, including object terms and either a predicate or existential quantifier.

Flatten:

Restate without nesting.

Functional fact type:

Fact type with a functional role.

Functional role:

Role with a simple uniqueness constraint.

Generalization:

Forming a more general case from one or more specific cases; the inverse of specialization.

Independent object:

Object that may exist without participating in any elementary fact; the disjunction of fact roles played by an independent object type is optional.

Instance:

An individual occurrence (one specific member of a type).

Mandatory role:

Role that must be played by all instances in the population of the object type playing the role; also called a total role.

Modality:

Mode in which a proposition is expressed. In ORM 2, modalities are either alethic (expressing necessities or possibilites) or deontic (expressing obligations or permissions).

Nested object:

Relationship that plays some role (also called an objectified relationship).

Object:

Thing of interest; an object may be an entity or a value.

Objectification:

Treating a relationship as an object; also called nesting. Strictly speaking, objectification in ORM 2 distinguishes the object formed by the objectification from the original relationship, and hence involves situational rather than propositional nominalization.

Object-Role Modeling (ORM):

Conceptual modeling method that pictures a business domain in terms of objects playing roles; it provides graphical and textual languages for verbalizing and querying information as well as various design and transformation procedures.

Population:

Set of instances present in a particular state of the database.

Predicate:

Proposition with object-holes in it, e.g. “... works for ...”.

Reference:

Relationship used as the preferred way to reference or identify an object (or to provide part of the identification).

Reference mode:

Mode or manner in which a single value references an entity; used to abbreviate simple reference schemes, e.g. (.code), (kg:).

Reference role:

Role in a reference (existential fact).

Relationship:

Property or association involving one or more objects.

Rigid subtype:

Subtype whose instances must remain in that type for their whole lifetime (e.g, Person).

Rmap:

Relational mapping procedure.

Role:

Part played by an object in a relationship (unary, binary, ternary, etc.).

Role subtype:

Subtype whose instances may leave that type during their lifetime (e.g., Child).

Semiderived fact type:

Fact type, some of whose instances may be derived from others, while some other instances may be simply asserted.

Semiderived subtype:

Subtype, some of whose instances may be derived using a derivation rule while some other instances may be simply asserted.

Subtype:

Object type that is properly contained in another object type (e.g., Woman is a subtype of Person).

Type:

Set of possible instances.

Uniqueness constraint (UC):

Repetition is not allowed in the role or role sequence spanned by the constraint; a uniqueness constraint on a single predicate is an internal UC, and a uniqueness constraint over roles from different predicates is an external UC.

Universe of Discourse (UoD):

Business domain (the aspects of the world that we want to talk about).

Value:

Unchangeable object that is identified by a constant; in this book a value is either a character string or a number; sometimes called a label.

The following symbol glossary covers the main graphical symbols in ORM 2 (as supported by the NORMA tool) and the corresponding symbols in ORM 1 (as supported in Microsoft Visio for Enterprise Architects).