[bkoelman]

Due to limitations in SQL Server, the default delete behavior for nullable foreign keys in Entity Framework Core is ClientSetNull instead of SetNull. This means that EF Core sends UPDATE statements to set all related foreign keys to null before sending the DELETE statement. But that only works when all related entities are loaded in the change tracker upfront.

For example, consider the following model:

[Resource]
public class Order : Identifiable<long>
{
}

[Resource]
public class Customer : Identifiable<long>
{
    [ForeignKey("FirstOrderId")]
    public Order? FirstOrder { get; set; }

    [HasOne]
    [ForeignKey("LastOrderId")]
    public Order? LastOrder { get; set; }
}

When sending DELETE /orders/1, this PR ensures that all customers with FirstOrderId or LastOrderId = 1 get loaded in the change tracker before deleting the order. The missing [HasOne] on FirstOrder is intentional: this also works for relationships that are not publicly exposed.

All that side-loading, which only happens when ClientSetNull is used, is of course highly inefficient. Other databases, such as PostgreSQL and MySQL, don't need this workaround. We've documented this for a long time.

This PR adds validation of the Entity Framework Core mappings. It fails when ClientSetNull is used (explicitly or implicitly) with databases other than SQL Server. It also validates that no Identifying Foreign Keys are used, which break JSON:API semantics. By default, an exception is thrown at startup, which can be downgraded to logged warnings by setting options.ThrowForProblematicEntityMappings = false.

DapperExample was similarly updated to fully support nullable foreign keys when deleting a resource.

Note

The ClientCascase delete mode (also only needed for SQL Server) remains broken. Supporting that means we need to load all related entities recursively in the change tracker. It is never used by default in Entity Framework Core. For databases other than SQL Server, use Cascade instead. When using SQL Server, send JSON:API requests to delete related resources first to avoid a foreign key constraint error.

Fixes #1118.

QUALITY CHECKLIST

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

[codecov]

Codecov Report

❌ Patch coverage is 95.13889% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.14%. Comparing base (ebdd586) to head (ea6a6be).

Files with missing lines	Patch %	Lines
...xamples/DapperExample/Repositories/DapperFacade.cs	58.33%	4 Missing and 1 partial ⚠️
...Configuration/EntityFrameworkCoreModelValidator.cs	95.12%	2 Missing and 2 partials ⚠️
...les/DapperExample/Repositories/DapperRepository.cs	57.14%	2 Missing and 1 partial ⚠️
...TranslationToSql/DataModel/BaseDataModelService.cs	93.75%	0 Missing and 1 partial ⚠️
...Core/Repositories/EntityFrameworkCoreRepository.cs	98.11%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2090      +/-   ##
==========================================
+ Coverage   92.03%   92.14%   +0.11%     
==========================================
  Files         364      366       +2     
  Lines       11898    12079     +181     
  Branches     1995     2032      +37     
==========================================
+ Hits        10950    11130     +180     
+ Misses        586      585       -1     
- Partials      362      364       +2     

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