ORM (Object-Relationship Model) Tutorial
This is a hypertext tutorial for the ORM (Object-Relationship Model)
The ORM is the "static" part of OSA. That is, the ORM describes the "database"
of a model: what objects may exist, what object classes they belong to,
and what relationships exist between objects.
ORM Concepts and Terms
An ORM diagram is composed of several components which are described below.
An object is any unique person, place, or
thing, either physical or conceptual. It is represented by a black
Every object is inherently unique. For example, an object
can have more than one name, and the same name may be used for
more than one object. (See
synonyms and homonyms.)
Formally, every object is a
constant singleton object
Objects with common properties can be grouped into sets called
object classes. An object class is
represented by a rectangle containing the name of the object class.
Unlike most object-oriented
languages, object classes in OSA are sets. Thus, an object
may join or leave an object class.
Lexical Object Classes
A lexical object class is an object class
in which each member has a one-to-one mapping with its representation.
It is represented by a dashed rectangle containing the name of the class.
Relational Object Classes
A relational object class is
an object class whose members are relationships.
It is represented by a rectangle containing a
High-Level Object Classes
A high-level object class is an
object class that contains other object classes, relationship sets,
constraints, and notes. It has an exploded and an imploded
A relationship is a statement of
some connection between objects. It is represented by a line connecting the
objects, labeled with the relationship name.
Relationships can have arbitrary arity. That is, a relationship can occur
between two, three, four, or more objects simultaneously.
Relationships between objects in object classes can be grouped
into relationship sets. A relationship
set is represented by a line connecting the object classes, labeled with
the relationship set name.
Special Relationship Sets
The following types of relationships can all be represented by normal
relationship sets, but they occur often enough that short hand notations
are defined for them:
High-Level Relationship Sets
A high-level relationship set
is a relationship set that contains other relationship sets, object
classes, constraints, and notes. It has an exploded and an
Constraints can be imposed on various parts of an ORM diagram. OSA includes
several types of constraints.
Object Class Cardinality Constraints
The number of objects in an object class can be constrained by
object-class cardinality constraints
(also called simply cardinality constraints).
A participation constraint determines
how many times each object in an object class can participate in a
given relationship set.
We use co-occurrence constraints in
relationship sets with arity of three or more. They specify how
many different objects can appear with a given object or group of objects
in a relationship set.
Specialization constraints qualify the
relationship between a generalization and
its specializations. A specialization constraint can be a
union constraint, a mutual exclusion constraint, a
an intersection constraint.
A general constraint expresses
constraints which are not otherwise
expressible. General constraints are often written in English, but
they can be expressed in more formal terms.
A note is information which may prove helpful
to people reading an ORM diagram. Semantically, it has no significance.
Go to the
Go to the
Begin the ORM tutorial with Object
Last updated 30 Sept 1994.
by Brent Goodman