[OASIS Issue Tracker] (ODATA-1409) Add support for joining entities with instances of their related collections

Gerald Krause created ODATA-1409:
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             Summary: Add support for joining entities with instances of their related collections 
                 Key: ODATA-1409
                 URL: https://issues.oasis-open.org/browse/ODATA-1409
             Project: OASIS Open Data Protocol (OData) TC
          Issue Type: Improvement
          Components: Data Aggregation
    Affects Versions: V4.0_CSD03
         Environment: Proposed
            Reporter: Gerald Krause
             Fix For: V4.0_CSD04


Such a feature would allow to handle property paths with one or more collection-valued segments in it, i.e. collection-valued navigation or complex properties.

There are use cases in data aggregation that require to deal with such paths, e.g. to group by properties that can only be reached via a path involving a collection-valued navigation property like this:

{{GET ~/Products?$apply=groupby((Name,Sales/Customer/ID), aggregate(TaxRate with average as AverageTaxRate))}}

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ODATA-1206 declared support for such paths with collection-valued properties in groupby, however it lacks a description how to process them.

This gap could be closed by a new transformation {{join}} that performs a disaggregation of the collections on the property path. The above request would then read:

{{GET ~/Products?$apply=join(Sales)/groupby((Name,Sales/Customer/ID), aggregate(TaxRate with average as AverageTaxRate))}}



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