Modeling Relationships

1. Modeling Relationships Farrokh Alemi, Ph.D.

2. Definition of Hierarchy • A collection of super and sub entities. • A super entity is the broadest definition of several sub entities. • A sub entity is an entity that inherits its relationship from another entity.

3. Definition of Relationship • Relationship describes the link between two entities • Different words for the same idea • At the conceptual level (e.g. entity and their relationships) • At the physical level (tables, data classes and linkages)

4. Relationships Are Identified Through Sentences • A sentence containing the names of the two entities and a verb phrase in between.  • Verb phrases:  "has," "contains," "visits," "prescribes," "travels with," etc. • The two entities "Patient" and "Clinician" can be made into a sentence such as "A patient visits a clinician." 

5. Relationships Revealed • Is one of the entities a look up table used to provide menu items for an attribute of the other entity.  • Is one of the entities a subcategory to another.  • Do two entities share an attribute. 

6. Relationships Documentation • The name of first entity • The name of second entity • The verb phrase describing the relationship • The cardinality of the relationship • one to one, one to many or many to many 

7. Some Relationships Suggest New Entities • Hierarchical • Many to many relationships

8. Generalization & Specialization • The process of abstracting from  narrowly scoped terms into terms of broader scope is a generalization process.   • Starting with a general term and narrowing its scope is a specialization process.

9. An Example of Generalization & Specialization

10. An Example of Generalization & Specialization

11. Hiearchy May Simplify Data Structures • Specializations of the more general data class Person • Oftentimes, introducing a subtype hierarchy can simplify the information model

12. Rules for Identifying Hierarchies • Super-type is the broadest entity,  should have all the attributes that are shared across the sub-entities.   • Each sub-entity should have an attribute that makes it different from other  entities.  • The attributes should be mutually  exclusive. 

13. Rules for Identifying Hierarchies

14. Confusion • It may not be possible for sub types to unambiguously reside in one  and only one of the subtypes specified in the hierarchy.  • Refine further • If confusing, do not do it

15. Steps for Identifying a Hierarchy • Shared attributes • Discriminator attribute • Entity linkages at the super type level

16. Step two: Discriminator

17. Example

18. Step Three: Linkages at Super Entity Level • Reduces unnecessary linkages • Simpler to read • Easier to implement

19. Advantages of Hierarchies Stabilize  the overall model with respect to new requirements.

20. An example New Requirement: “Patient of record” becomes inactive but the system does not purge it

21. Insert a new entity in the  hierarchy • Include the new entity in the discriminator attribute • Check that all sub entities are appropriately named

22. New Data Structure

23. Check Sub Entities The first issue is whether the sub entity is distinct form other entities 

24. Check Super Entity • Do the attributes in the super entity apply equally well to the new sub entity

25. Check Discriminator The third issue is whether the discriminator attribute in  the super-entity applies to all sub types The importance of  good entity and attribute definitions

26. Many to Many Relationships

27. Another Example

28. Patient Clinician Association

29. Association Class Simplifies • Resolve  all many-to-many linkages • A simpler way of capturing and tracking many relationships

30. Self Referral • A Person to Person Association • A self referring relationship is found in the same way as all relationships are found by making a sentence containing the name of the entity and a verb phrase

31. Portions of Information Model

32. New Requirements • Some of the facilities may have  multiple postal addresses. • One way to incorporate the new requirement would be to add  multiple attributes to each one of the entities. 

33. Revised Information Model

34. Take Home Lessons • We saw that scenarios focus on decisions • Scenarios contain use cases which reveal the information exchange • Exchanges are used to specify fields.