Cosmos: Add support for IS_DEFINED() to EF.Functions

[divega]

The full context is explained in #13131, but I decided to create a new issue to gather more targeted customer feedback on this.

The short version is that in a schema-less database like Cosmos DB it is valid for each document in a container to contain completely different set of properties. In an object mapping situation this translates into some of the mapped properties being absent. Cosmos DB provides a function IS_DEFINED() that can be used in queries to test for this situation. Note also that a property not being defined is different (albeit one can argue only slightly) from the value being null.

The Cosmos DB provider could define an extension method in EF.Functions that could be used, for example, like this:

var itemsMissingData = context.Items.Where(i => !EF.Functions.IsDefined(i.Data));

[divega]

It seems that Cosmos DB is still filtering out documents where a property is undefined if you ORDER BY that property. This makes providing access to the IS_DEFINED() function from LINQ queries more important. That way, you can use it like this:

var orderedItems = context.Items.OrderBy(i => i.Data);
var itemsMissingData = context.Items.Where(i => !EF.Functions.IsDefined(i.Data));
orderedItems.AddRange(itemsMissingData);

[smitpatel]

Also consider Coalesce operator ?? which is used to coalesce based on present keys.

[prachita-mane]

Do we have a timeline on when this will be implemented?