Root cause: synth litters (from chart parentRefs) were added with FatherId/MotherId pointing
to gerbils that were also [Added] in the same SaveChanges batch. With 337 derived litters from
41 pedigrees, multi-generational references created a true dependency cycle:
Gerbil [Added] ← FK{MotherId} Litter [Added] ← FK{LitterId} Gerbil [Added]
EF's topo-sort threw InvalidOperationException; smaller datasets (REIMPORT-1/2) didn't
trigger it because they had fewer chart-derived litters.
Fix: add synth litters with FatherId=null/MotherId=null (deferred), track intended FKs
in synthLitterPendingParents, apply them in a separate SaveChanges after all gerbils are
persisted. Single-direction Gerbil→Litter dependency in the main save; no cycle possible.
No schema change, no migration.
Regression test: artificial A←B←A cycle in SQLite (FK-enforced) must not throw.
Also verifies deferred FK update sets litter parent correctly after execute.
GATE: 158/158 C# tests, has-pending-model-changes=No
Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>
Gulum
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