Fail on missing required non-nullable value type in request body

[bkoelman]

This proof-of-concept PR fails deserialization of an incoming request body at a POST resource endpoint when the value is omitted for a non-nullable value type property decorated with [Attr] [Required].

This works around the limitation in ASP.NET ModelState validation described at https://www.jsonapi.net/usage/resources/nullability.html#value-types.

However, I wonder if we should actually take this approach. It feels like a dirty hack that mixes model binding with validation, which are distinct steps in ASP.NET. Also, it prevents any other ModelState violations from being returned in the response. A better approach would be to still detect it during deserialization, but instead of producing an error, somehow make it result in a ModelState validation error via request-scoped shared state. This would only work properly when ModelState validation is turned on. That approach would potentially apply to various other errors that currently cause a deserialization failure, such as "missing ID", "property is read-only" etc.

And this is just one approach to dealing with the situation. Alternatives:

• Document the behavior and advise users not to do this. This is a fundamental limitation in ASP.NET, not our concern.
• Scan for this situation when building the resource graph and produce a hard error that prevents application startup, or log a warning. The former is a breaking change, the latter means it'll still occur and we may want to handle it.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• TODO: Adapted tests
• Documentation updated

[codecov]

Codecov Report

Merging #1254 (18407ce) into data-types (78d753c) will decrease coverage by 0.15%.
The diff coverage is 50.00%.

@@              Coverage Diff               @@
##           data-types    #1254      +/-   ##
==============================================
- Coverage       92.68%   92.53%   -0.15%     
==============================================
  Files             243      244       +1     
  Lines            7804     7827      +23     
==============================================
+ Hits             7233     7243      +10     
- Misses            571      584      +13     

Impacted Files	Coverage Δ
...zation/Request/JsonMissingRequiredAttributeInfo.cs	0.00% <0.00%> (ø)
...lization/JsonConverters/ResourceObjectConverter.cs	94.05% <66.66%> (-2.68%)	⬇️
...lization/Request/Adapters/ResourceObjectAdapter.cs	97.29% <77.77%> (-2.71%)	⬇️
...ore/Controllers/BaseJsonApiOperationsController.cs	100.00% <0.00%> (ø)

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[bkoelman]

Closing due to #1245 (comment).