Allow Singletons to be null

The proposal is to allow a Singleton to be specified as Nullable. In clause 13.3 Singleton, add: Attribute Nullable The value of Nullable is one of the Boolean literals true or false. Absence of the attribute means false. A value of true specifies that the declaring Singleton MAY have no related entity. If false, instances of the declaring Singleton MUST always have a related entity. Behavior: For 4.01 responses: -services return "nullable":true for nullable singleton in JSON. absence means not-nullable, as with all other nullable facets in JSON. -services return "nullable=true" for nullable singleton in XML. Absence of this attribute means not-nullable. note that this is the opposite default as for single-valued navigation properties. -services return 204, No Content, for null-valued singletons (just as for null-valued singleton navigation properties). For 4.0 responses: -services must return "nullable":false, or omit the nullable facet, for a singleton in JSON -services must not return the nullable attribute for a singleton in XML -services must return a representation of the singleton (i.e., an object with only a context Url) or an error (such as 404).

https://github.com/oasis-tcs/odata-csdl-schemas/pull/2 https://www.oasis-open.org/committees/download.php/64129/odata-csdl-json-v4.01-wd05-2018-10-22.docx   https://www.oasis-open.org/committees/download.php/64130/odata-csdl-xml-v4.01-wd06-2018-10-22.docx