Migrations

Pipeline

Add migration: MigrationsScaffolder.ScaffoldMigration() → MigrationsModelDiffer.GetDifferences() → list of MigrationOperation → CSharpMigrationsGenerator and CSharpSnapshotGenerator produce Up/Down/Snapshot code

Apply migration: Migrator.MigrateAsync() → reads __EFMigrationsHistory → per pending: MigrationsSqlGenerator.Generate(operations) → MigrationCommandExecutor executes

Model Snapshot

• Model snapshots use typeof(Dictionary<string, object>) (property bag format), not the actual CLR type. When examining the ClrType in a snapshot, don't assume it matches the real entity type.
• SnapshotModelProcessor.Process() is used at design-time to fixup older model snapshots for backward compatibility.
• MigrationsModelDiffer uses provider-agnostic structural comparison between relational models to determine what migration operations are necessary.

For an upgrade-only regression, derive a concrete old ModelSnapshot, process its model with SnapshotModelProcessor.Process(..., resetVersion: true), and compare it with the equivalent current design-time model through MigrationsModelDiffer.GetDifferences(). Preserve the property-bag CLR types emitted by old snapshots and omit APIs that would explicitly configure metadata that was absent in that version. The regression succeeds only when the resulting operation list is empty.

Legacy snapshot normalization belongs in SnapshotModelProcessor, not in runtime migration application. Scope a compatibility branch to the exact released versions that could emit the affected snapshot shape; versions from before the feature existed should not be included merely because they are numerically older. Prefer the mutable metadata interface when it exposes the required setter rather than casting to a concrete metadata implementation. Do not use a missing configuration source as the test for missing historical snapshot metadata: old snapshots may already configure a value using the current default. For metadata whose semantics changed, use the snapshot's product version to restore the old invariant. Add focused coverage for every version family named by the gate, plus the first unaffected version.

Testing

Migration operation tests: test/EFCore.Relational.Tests/Migrations/. Functional tests: test/EFCore.{Provider}.FunctionalTests/Migrations/. Model differ tests: test/EFCore.Relational.Tests/Migrations/Internal/MigrationsModelDifferTest*.cs.

To simulate a snapshot model use ModelBuilder calls without conventions.