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What is a migration in EF Core?

Entity Framework & EF Core Modelling Basic 1–2 Yrs2–3 Yrs Conceptual 2 min read

Quick Answer

A migration is a versioned, code-generated description of a schema change. When you change your entity model, dotnet ef migrations add <Name> diffs the model against the last migration and writes Up()/Down() methods. dotnet ef database update (or Migrate() / a SQL script) applies them, and EF records which have run in the __EFMigrationsHistory table.

A migration is EF's record of a schema change, in code. I change the model, run migrations add with a name, and EF diffs against the last snapshot and generates Up and Down methods — create table, add column, and so on. Then database update applies it and EF logs it in the __EFMigrationsHistory table so it won't run twice. In development I just run update; for production I generate an idempotent SQL script and run it through the deployment pipeline. The rules are: review the generated migration for data loss and renames, never edit one that's already been applied, and keep migrations backward-compatible if you want zero-downtime releases.

Workflow:

  1. Change the model (add a property, a table, an index).
  2. dotnet ef migrations add AddOrderStatus — generates a migration class and updates the model snapshot.
  3. Review the generated code — check for data loss (dropped columns), rename detection (EF sees drop+add, not rename), and long-running operations.
  4. Apply: database update in dev; in production prefer dotnet ef migrations script (idempotent SQL) run through your deployment pipeline, or DbContext.Database.Migrate() at startup for simple apps.

Interview points: never edit an applied migration — add a new one; migrations should be additive/backward-compatible for zero-downtime deploys (expand-then-contract); the model snapshot must stay in source control; multiple developers adding migrations in parallel causes snapshot conflicts that must be resolved carefully.

CLI — EF Core migrations 
dotnet ef migrations add AddOrderStatus
dotnet ef database update            # apply in dev

# for production: generate a script to run through the pipeline
dotnet ef migrations script --idempotent --output migrate.sql
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