Document obsoletion of owned JSON collections without an explicit key (EF11)

[Copilot]

Per dotnet/efcore#38395, EF Core 11 raises an OwnedEntityMappedToJsonCollectionWarning when an owned type is mapped to JSON as a collection without an explicit primary key (synthetic ordinal key). The original broader deprecation proposed in dotnet/efcore#37290 was narrowed to just this case.

Changes

• entity-framework/core/what-is-new/ef-core-11.0/breaking-changes.md
  • New low-impact entry #owned-json-collections-obsolete following the standard Old/New/Why/Mitigations template.
  • Added corresponding row to the summary table.
  • Recommended mitigation: migrate to complex-type JSON mapping (the preferred mapping since EF 10). Fallback: configure an explicit key on the owned type, or suppress the warning via ConfigureWarnings.

// Before — now raises OwnedEntityMappedToJsonCollectionWarning
modelBuilder.Entity<Blog>().OwnsMany(b => b.Posts, b => b.ToJson());

// Recommended
modelBuilder.Entity<Blog>().ComplexCollection(b => b.Posts, b => b.ToJson());

// Or keep OwnsMany with an explicit key
modelBuilder.Entity<Blog>().OwnsMany(b => b.Posts, b =>
{
    b.ToJson();
    b.HasKey(p => p.Id);
});

// Or suppress the warning via configuration
optionsBuilder.ConfigureWarnings(w => w.Ignore(CoreEventId.OwnedEntityMappedToJsonCollectionWarning));

The new anchor is the intended target for the existing aka.ms/efcore-docs-json-owned-entities redirect (redirect update handled separately).

[Copilot]

Pull request overview

Documents an EF Core 11 breaking-change/deprecation: owned types mapped to JSON collections without an explicit primary key now raise OwnedEntityMappedToJsonCollectionWarning, and the pattern is considered obsolete going forward.

Changes:

• Adds a new low-impact breaking-changes entry (#owned-json-collections-obsolete) using the standard Old/New/Why/Mitigations structure.
• Updates the breaking changes summary table to include the new entry.
• Provides mitigations: prefer ComplexCollection(...).ToJson(), or keep OwnsMany(...).ToJson() with an explicit key, or suppress via ConfigureWarnings.