Fix FK to entity-split principal referencing wrong (fragment) table

[Copilot]

Fixes#29915

When using SplitToTable, foreign keys from external entities to the split entity incorrectly referenced the last-added fragment table instead of the main table — e.g., FK_Management_Address_CompanyId instead of FK_Management_Company_CompanyId.

modelBuilder.Entity<Company>(e =>e.SplitToTable("Address", t =>{t.Property(x =>x.City);}));// FK on Management was generated pointing to Address instead of CompanymodelBuilder.Entity<Management>(e =>e.HasOne(d =>d.Company).WithMany(p =>p.ManagementSet).HasForeignKey(d =>d.CompanyId));

Root cause

In RelationalModel.PopulateForeignKeyConstraints, principal table mappings are iterated in reverse to prefer the most-derived table for TPT inheritance. For entity-split entities, tableMappings after AddTables is [main_table, fragment_table, ...], so reversing makes the fragment table the first candidate — and since a valid constraint name can be computed against any table that contains the PK column, the FK was created against the fragment table.

An additional consequence of the original fix (filtering out all fragment mappings) was that FKs targeting an alternate key defined on a fragment table were silently dropped, since the AK column only exists on the fragment.

Fix

• RelationalModel.PopulateForeignKeyConstraints: Non-fragment table mappings (IsSplitEntityTypePrincipal != false) are tried first (reversed, to preserve TPT inheritance preference). Fragment table mappings (IsSplitEntityTypePrincipal == false) are used as a fallback so that FKs targeting an AK that lives on a fragment are still resolved correctly. Non-split and TPT mappings all have IsSplitEntityTypePrincipal = null, so behavior for those cases is unchanged.
• MigrationsModelDifferTest: Added Foreign_key_to_entity_split_principal_points_to_main_table test covering the PK-targeting FK scenario, and Foreign_key_to_entity_split_principal_ak_on_fragment_points_to_fragment_table test covering the case where the FK targets an AK defined on a fragment table.

[Copilot]

Pull request overview

Fixes relational model FK constraint resolution for entity-split principals (SplitToTable) so that foreign keys from other entities (and the split fragment FK itself) target the principal’s main table rather than an arbitrary fragment table.

Changes:

• Filter principal table mappings in RelationalModel.PopulateForeignKeyConstraints to exclude split-fragment mappings (IsSplitEntityTypePrincipal == false) before applying the existing “prefer most-derived” reverse ordering.
• Add a regression test ensuring FKs to an entity-split principal point to the main table (not the fragment table).

Reviewed changes

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File	Description
src/EFCore.Relational/Metadata/Internal/RelationalModel.cs	Ensures FK principal table selection ignores split fragment mappings while preserving TPT/TPC behavior.
test/EFCore.Relational.Tests/Migrations/Internal/MigrationsModelDifferTest.cs	Adds coverage for the reported FK-to-fragment regression scenario.

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Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

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