The Data Model Resource Book, Volume 3: Universal Patterns for Data Modeling

Silverston c04.tex V2 - 11/21/2008 3:02am Page 178

178 Chapter 4 ■ Hierarchies, Aggregations, and Peer-to-Peer Relationships

with a WORK EFFORT ASSOCIATION RULE for these first three roles. In

other words, ‘‘Systems Analysis’’ has three child work efforts, ‘‘Create Initial

Scope Statement,’’ ‘‘Create Source System Inve ntory,’’ and ‘‘Discover Target

Requirements.’’ This structure can be classified as a ‘‘Work Breakdown Struc-

ture.’’ We did not assign any rule to the relationships between them. WORK

EFFORT ASSOCIATION RULE is not mandatory, and it is not necessary to

have a rule for every association.

If you look at the fourth row of the table you see that ‘‘Create Source System

Inventory’’ and ‘‘Discover Target Requirements’’ are of the association type of

‘‘Peer Work Efforts’’ and have a rule of ‘‘Concurrent,’’ meaning they need to be

performed at the same time. Instead of just saying that ‘‘Create Source System

Inventory’’ and ‘‘Discover Target Requirements’’ are related to each other as

‘‘Peer Work Efforts,’’ we are also storing the WORK EFFORT ASSOCIATION

RULE of ‘‘concurrent’’ to show that they must be done together.

You might ask, ‘‘Couldn’t you do this with the previous pattern, have

two rows: one association type for ‘‘Peer Work Efforts’’ and one for ‘‘Concur-

rency’’?’’ Yes, you could do this in the previous pattern. But are ‘‘Concurrency’’

and ‘‘Peer Work Efforts’’ the same thing? ‘‘Peer ...