# Migrations

> Implementation details for EF Core migrations. Use when changing MigrationsSqlGenerator, model diffing, migration operations, HistoryRepository, the Migrator or related classes.

- Skill: `dotnet/migrations` (Agent Skill)
- Install (CLI): `npx skillmds@latest add dotnet/migrations`
- Raw SKILL.md: https://api.skillmd.com/api/skills/dotnet/migrations/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: .NET (Microsoft) (https://skillmd.com/u/dotnet)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/dotnet/migrations

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# 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.

## 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`.