[OASIS Issue Tracker] (ODATA-1059) Improve specification and use of NavigationProperties, EntitySets, and Singletons

[OASIS Issues Tracker <workgroup_mailer@lists.oasis-open.org>]

     [ https://issues.oasis-open.org/browse/ODATA-1059?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

George Ericson updated ODATA-1059:
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    Labels: Extension Usability request_tc_discussion  (was: )

> Improve specification and use of NavigationProperties, EntitySets, and Singletons
> ---------------------------------------------------------------------------------
>
>                 Key: ODATA-1059
>                 URL: https://issues.oasis-open.org/browse/ODATA-1059
>             Project: OASIS Open Data Protocol (OData) TC
>          Issue Type: Improvement
>          Components: ABNF Construction Rules, CSDL JSON , CSDL XML, Implementing OData
>         Environment: Any
>            Reporter: George Ericson
>            Assignee: George Ericson
>              Labels: Extension, Usability, request_tc_discussion
>
> There is significant confusion over the role and semantics of Singletons and EntitySets.  For example, since Nullable is not an attribute, it is assumed that they must have a value, but implementors are also assuming that if a particular Singleton or EntitySet is not implemented, then a value need not be returned.
> Observations on Singletons and EntitySets from an OData metamodel point of view:
> 1) A Singleton is a scalar Property with a Type that is an EntityType.
> 2) An EntitySet is an array Property with a Type that is an EntityType.
> 3) A NavigationProperty with ContainsTarget="True" is a scalar or array Property with a Type that is an EntityType.
> 4) A NavigationProperty with ContainsTarget="False" is an association end with a Type that is an EntityType.
> The overlap of 1, 2, and 3 is a source of great confusion.  Additionally the reasons for variations in semantics of these metatypes are not clear.  (i.e. missing Nullable attribute.)
> The following proposals are alternatives for simplifying the OData metamodel.
> Proposal 1: 
> 1) Allow use of Singleton and EntitySet in the context of an EntityType or ComplexType declaration.
> 2) Deprecate the ContainsTarget attribute.
> 3) Add Nullable attribute to Singleton and EntitySet.
> The advantages of Proposal 1:
> 1) Eliminates implied Singleton and EntitySet.  These are now first class citizens in all cases.
> 2) Simplifies specifications since containment NavigationProperty is eliminated.
> 3) Clarifies whether or not a value may be omitted.
> Proposal 2:
> 1) Add EntityProperty, with same syntax and semantics as Property except that Type must be an EntityType.
> 2) Allow use of EntityProperty in the context of EntityContainer, EntityType, or ComplexType.
> 3) Deprecate the ContainsTarget attribute.
> 4) Deprecate Singleton and EntitySet
> 5) Modify syntax of NavigationProperty to only reference EntityProperties.
> The advantages of Proposal 2:
> 1) Eliminates Singleton and EntitySet.  These are now first class EntityProperty elements in all cases.
> 2) Eliminates need for ContainsTarget attribute and therefore the concept of containment NavigationProperty.
> 3) Rationalizes the syntax differences between Singleton, EntitySet, and NavigationProperty around Property.
> 4) Simplifies semantics of NavigationProperty as an association.
> 5) Clarifies whether or not a value may be omitted.
> Proposal 3:
> 1) Same as Proposal 2 except that instead of adding EntityProperty, instead extend Property to allow Type to be an EntityType.  In this case, the spec might refer to an entity property as a Property with Type that is an EntityType.
> 2) Allow use of Property in the context of EntityContainer, EntityType, or ComplexType declarations.
> 3) Deprecate the ContainsTarget attribute.
> 4) Deprecate Singleton and EntitySet
> 5) Modify syntax of NavigationProperty to only reference entity properties.
> The advantages of Proposal 3:
> 1) Everything in Proposal 2.
> 2) Does not add a new metatype. 



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