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Name	Symmetric n-ary relationship
Also known as
Author(s)	MariaPoveda, MariCarmenSuarezFigueroa
SubmittedBy	MariaPoveda, MariCarmenSuarezFigueroa

Motivation	The symmetric n-ary relationship pattern emerged from the need of modelling distances among two points. This problem is a clear case of an n-ary relationship where the relation between two points needs a further argument to represent the distance between such points. Once we have applied the n-ary pattern for this use case (http://www.w3.org/TR/swbp-n-aryRelations/#useCase1), we can realize that the origin and the destination of the n-ary relation belong to the same class. In addition, the value for the relationship is the same to represent the distance from A to B and vice-versa. Then, if we want to represent both distances we should instanciate the pattern twice, from A to B and vice-versa, resulting in a redundant representation.
Aim	This pattern allows representing symmetric n-ary relationships, i.e. binary relationships between two elements that need a further argument that has the same value for both directions of the relationship. If SNAry is the symmetric n-ary relationship and z is its value for the elements x and y, then: SNAry(x,y)=z iff SNAry(y,x)=z
Solution description	A class to represent the n-ary relationship together with the value for the further needed argument (Relationship or Attribute) has been created. A relationship between the abovementioned class and the classes involved in the symmetric n-ary relationship is created.
Elements	Class, Relationship, Attribute Axioms: cardinality and equivalentClass
Implementation
Reusable component
Component type

Problem example	For example, we might want to represent the distance expressed in kilometres between two places.
Pattern solution example	http://ontologydesignpatterns.org/wiki/images/b/bd/Symmetric_n-ary_relationship_distance.jpg
Consequences	The main advantage of this pattern is that allows representing distance between places without including redundancy in the ontology.

Origin	Modelling distance between stages of St James Way during the Geobuddies ontology network development (http://geobuddies.dia.fi.upm.es).
Known use	Geobuddies ontology network
Reference
Related ODP	N-ary Relation: New Class (LP-NR -01)
Used in combination with
Test

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	Submission to event WOP:2010

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