$hierarchy

[HeikoTheissen]

To simplify the syntax of hierarchical transformations, remove the first three parameters $H,Q,I$ of the hierarchical transformations ancestors, descendants, traverse and of rolluprecursive and introduce a new system query option $hierarchy=$H,Q,I$. The third parameter $I$ can be omitted if it equals the @Aggregation.RecursiveHierarchy#Q/NodeProperty.

• Ensures there is no "switch" in the hierarchy between descendants and a subsequent ancestors.
• Avoids redundancy (see this test case)
• Further abbreviations are possible:
  • $hierarchy=SalesOrgHierarchy could imply hierarchy collection = requested resource.
  • If there is only one Aggregation.RecursiveHierarchy annotation (perhaps even unqualified?) on the requested resource, $hierarchy need not be specified at all.

Without the redundancy, it seems justifiable to allow full-blown hierarchical transformations as optional parameters of rolluprecursive (#86):

SalesOrganizations?$apply=
descendants(filter(ID eq 'US'), keep start)
/groupby(
  (rolluprecursive(ancestors(filter(contains(Name, 'New York')), keep start))),
  aggregate(...)
)&$hierarchy=$root/SalesOrganizations,SalesOrgHierarchy

If the requested entity set is Sales instead of SalesOrganizations, however, the hierarchical transformations nested in rolluprecursive always use the node property path specified in the RecursiveHierarchy annotation, regardless of what is specified in the third parameter of the $hierarchy option.

Sales?$apply=
descendants(filter(SalesOrganization/ID eq 'US'), keep start)
/groupby(
  (rolluprecursive(ancestors(filter(contains(Name, 'New York')), keep start))),
  aggregate(...)
)&$hierarchy=$root/SalesOrganizations,SalesOrgHierarchy,SalesOrganization/ID

A similar switch in the node property path happens in connection with transformnested.

As an alternative to the system query option $hierarchy, we could introduce a transformation hierarchy$(H,Q,I,T)$ where $T$ is a transformation sequence.