About Cardinalities

latest update: 2019-01-26    


In ISO 15926 the definition of cardinalities is exactly the opposite of the customary definition.

This topic explains why and how.

Cardinalities - customary

Let us take the relationship between a CENTRIFUGAL PUMP and IMPELLER.

Normally a Cardinality defines the minimum and maximum number of occurrences of one entity for a single occurrence of a related entity.

So a CENTRIFUGAL PUMP in general has one to many IMPELLERs, and when we specialize that to a TWO-STAGE CENTRIFUGAL PUMP it has two IMPELLERs.

In these diagrams a relation is shown.

Cardinalities - ISO 15926

In ISO 15926 relationships are understood to be classes themselves that HAVE, not ARE, relations:

  • Relationship - with two relations
  • MultidimensionalObject - with two to many relations.

Let's focus on such Relationships:


This diagram shows that myPump shall be involved in exactly two member Relationships at any point in time and myImpeller#1 may be part of myPump, the same holds for myImpeller#2.

dm:AssemblyOfIndividual is a specialization of dm:Relationship that is a member of the dm:ClassOfAssemblyOfIndividual, where the latter is a specialization of dm:ClassOfRelationship.

ClassOfRelationship, and all its specializations, has an end1Cardinality and an end2Cardinality relation to dm:Cardinality.

So, here the cardinality is in fact attributed to the ClassOfRelationship and not to the related things.