[OASIS Issue Tracker] (ODATA-1092) Navigation Property Binding (public comment c201707e0004)

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Ralf Handl created ODATA-1092:
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             Summary: Navigation Property Binding (public comment c201707e0004)
                 Key: ODATA-1092
                 URL: https://issues.oasis-open.org/browse/ODATA-1092
             Project: OASIS Open Data Protocol (OData) TC
          Issue Type: Task
          Components: CSDL JSON , CSDL XML
    Affects Versions: V4.01_CSD02
            Reporter: Ralf Handl
             Fix For: V4.01_CS01


Public Review Comment https://lists.oasis-open.org/archives/odata-comment/201707/msg00004.html

This comment concerns navigation property bindings.  There are two parts, a general issue with 4.0 and a similar issue that has been introduced with 4.01.  The comment refers to:

OData Common Schema Definition Language (CSDL) XML Representation Version 4.01
Committee Specification Draft 02 / Public Review Draft 02

§ 8.4 says:

> Containment navigation properties define an implicit entity set for each instance of its declaring structured
> type

§ 13.4 says:

> If the entity type of an entity set or singleton declares navigation properties, a navigation property binding
> allows describing which entity set or singleton will contain the related entities

It therefore makes sense that a Binding Target must identify a single entity set.

My issue is that this path is allowed to be a path to any "containment navigation property in scope".  Such a path only identifies an entity set uniquely in cases where the path includes a singleton.  If the path includes an EntitySet then, as per 8.4, it will be pointing to a set of EntitySets, one per entity instance.  

§ 15.4 contains an example that highlights the problem:

> MySchema.MyEntityContainer/MyEntitySet/MyContainmentNavigationProperty

This issue applies to both version 4.0 and 4.01.  The resulting navigation property is only weakly bound.  Anyone attempting to validate a link must exhaustively search every instance of MyEntitySet to determine the validity of the link.  You also have the strange situation that a bound navigation property may link to two different entities WITH THE SAME KEY.  (There is no requirement that keys are unique across entity sets.) This contravenes one of the methods of addressing entities in URLs:

OData Version 4.01. Part 2: URL Conventions § 4.9

> For [ ... ] collection-valued navigation properties with a NavigationPropertyBinding or ContainsTarget=true specification, members of the collection can be addressed by convention by appending the parenthesized key to the URL specifying the collection of entities

So the above is already a problem IMO and I propose that the target paths be restricted to traversing Singletons (that is, if an entity set is specified it must be the last component of the path).

Coming on to the new issue in 4.01...

Version 4.01 has modified the way navigation bindings work to allow a single navigation property to be bound, simultaneously, to multiple entity sets based on the type of the target.  Straight away this triggers the same issue, that bound navigation properties no longer have unique keys.

I'm unhappy with the idea that a binding may now bind to multiple entity sets as it would involve removing the ability to append a key to uniquely identify an entity via a (bound) navigation path.  It isn't clear what problem you're trying to solve here but it feels like it is best solved using multiple navigation properties rather than attempting an 'octopus binding'.

Even if you allow a single bound navigation property to bind to multiple entity sets the new feature creates the possibility of a partially bound navigation property.  If I have a navigation property called A of type Collection(TypeA) and TypeA has two sub-types, TypeB and TypeC then we can now bind any of the following:

A
A/TypeA
A/TypeB
A/TypeC

The most specific rule applies so the last two rules override the first two but if we only bind A/TypeC then any instances of TypeB linked to A are unbound.  The result is a partially bound navigation property.  This could be corrected by requiring a default binding (with no type cast segment) if a type-cast binding is provided.

Hopefully helpful.

Steve



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