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

Silverston c04.tex V2 - 11/21/2008 3:02am Page 143

Level 1 Recursive Pattern 143

implies ownership, and if the ‘‘owning’’ object is destroyed, the ‘‘owned’’

objects or instances are also destroyed.

Some interesting i ssues crop up because of the structure seen in Figure 4-5.

The estimated hours attribute gets enumerated at all three levels. For ex-

ample, the PROJECT has estimated hours of ‘‘2300’’ hours, the total number of

PHASE estimated hours is ‘‘1410’’ hours (‘‘700’’+‘‘400’’+‘‘200’’+‘‘110’’), and

finally, the summation of all TASK estimated hours works out to be ‘‘1110’’

hours. This is a s trength and a weakness of this pattern. Each of the difgferent

estimates may be independent of each other. You can define the estimated

hours for a PROJECT to be different from the estimated hours for a PHASE. For

example, at the beginning of the project you may do the project estimate; then as

you progress through each phase, you may estimate the hours on a phase level

and so on. If you wanted to maintain hours only at the TASK level, you could

delete the estimated hours attributes from PHASE and PROJECT. In other

words, the Level 1 Recursive Pattern allows you to capture specific attributes at

any level in any hierarchy/aggregation. This means that you can support very

specific business rules about the hierarchy or aggregation. In our example, we

couldhaveincludedapercentage complete attribute at only the ...