Ef Core Migration Safety

EF Core Migration Safety

Classify every migration before merging

1. Additive online - new nullable column, new table, new index (if built concurrently). Safe to deploy in any order.
2. Additive blocking - new NOT NULL column without default on a large table, new index without CONCURRENTLY (Postgres) / ONLINE = ON (SQL Server). Locks the table for the duration.
3. Destructive - drop column/table, narrow a type, add a constraint existing rows violate, rename. Requires the expand/contract pattern below.

Grep the generated migration for DropColumn, DropTable, AlterColumn, RenameColumn, AddColumn with nullable: false. Any hit means the migration cannot be reviewed by skimming the model diff - read the SQL via dotnet ef migrations script.

Renames are drops in disguise

EF cannot always tell a rename from drop+add. Verify the migration contains RenameColumn, not this:

// non-compiling: illustrative
// WRONG: silently destroys data
migrationBuilder.DropColumn(name: "Surname", table: "Users");
migrationBuilder.AddColumn<string>(name: "LastName", table: "Users");

If the scaffolder produced drop+add, hand-edit it to RenameColumn. Test by applying to a database with data, not an empty one.

Zero-downtime column changes (expand/contract)

Old app code and new schema coexist during a rolling deploy. Never ship a migration the previous app version cannot run against.

Adding a required column:

// Release 1: add nullable, app writes it
migrationBuilder.AddColumn<string>("Region", "Orders", nullable: true);
// Release 2: backfill out-of-band, then enforce
migrationBuilder.Sql("UPDATE Orders SET Region = 'EU' WHERE Region IS NULL");
migrationBuilder.AlterColumn<string>("Region", "Orders", nullable: false);

Dropping a column: release 1 removes every read and write of the property but keeps it mapped, so old and new app versions both run against the existing column; release 2 removes the property from the model and ships the DropColumn migration. A one-release drop breaks the still-running old instances.

NOT NULL with defaultValue on big tables

migrationBuilder.AddColumn<bool>("IsActive", "Users", nullable: false, defaultValue: true);

On SQL Server 2012+/Postgres 11+ this is metadata-only. On older engines it rewrites the table under an exclusive lock. Know your target before approving.

The shadow-property trap

An FK without a navigation-configured principal or a misspelled property gets a shadow column (CustomerId1). Symptoms: a duplicate-looking column in the migration, silently null FKs at runtime. When a migration adds a column you did not add to the model, stop - it is almost always a relationship EF inferred wrongly. Fix the model configuration; never merge the migration hoping it is harmless.

Operational rules

• Never call Database.Migrate() from application startup in multi-instance deployments: concurrent migrators race, and a failed migration takes the app down. Run migrations as a separate deploy step (dotnet ef database update or an idempotent script with --idempotent).
• Generated Down() methods restore schema, not data. Treat rollback of destructive migrations as impossible; roll forward instead.
• Migrations are immutable once merged to main. Fix mistakes with a new migration, never by editing an applied one - the __EFMigrationsHistory hash will not match and other environments diverge.
• Squash only migrations that no environment has partially applied.
• Review raw SQL in migrations (migrationBuilder.Sql) for idempotency: it runs exactly once per database, but --idempotent scripts may replay surrounding context.