Migrations

Level: Beginner

ℹ️ Where This Fits

Migrations track schema changes over time. This enables version control of your database, enabling teams to collaborate and deploy safely.

ℹ️ What You'll Learn

• Migration purpose: Track database schema changes as code (version control friendly)
• Workflow: Modify entity → add migration → review SQL → update database
• dotnet ef migrations add: Create new migration file based on model changes
• dotnet ef database update: Apply pending migrations to database
• Automatic timestamping: Migration files timestamped (202401151430_AddStudentTable.cs)
• Up method: SQL to apply migration (ALTER TABLE, CREATE TABLE, etc)
• Down method: SQL to revert migration (rollback)
• Migration history: __EFMigrationsHistory table tracks applied migrations
• Idempotent scripts: Migrations check if already applied (safe to re-run)
• Removing migrations: dotnet ef migrations remove (only before applying)
• Script-only: dotnet ef migrations script (generates SQL without applying)
• School Management migrations: Initial, AddStudentTable, AddExamResults, AddFeePayments
• Common mistakes: Modifying migration files manually, removing applied migrations, losing down methods

Basic Migration Workflow

# 1. Create migration (generates SQL based on model changes)
dotnet ef migrations add InitialCreate

# 2. Apply migration to database
dotnet ef database update

# 3. View generated SQL (without applying)
dotnet ef migrations script

Migration File Example

Generated file: Migrations/20240115143045_InitialCreate.cs

public partial class InitialCreate : Migration
{
    protected override void Up(MigrationBuilder migrationBuilder)
    {
        migrationBuilder.CreateTable(
            name: "Students",
            columns: table => new
            {
                Id = table.Column<int>(nullable: false)
                    .Annotation("SqlServer:Identity", "1, 1"),
                Name = table.Column<string>(maxLength: 100, nullable: false),
                ClassName = table.Column<string>(maxLength: 10, nullable: false),
                RollNumber = table.Column<string>(maxLength: 20, nullable: false),
                DateOfBirth = table.Column<DateTime>(nullable: false),
                Status = table.Column<int>(nullable: false),
                CreatedAt = table.Column<DateTime>(nullable: false)
            },
            constraints: table =>
            {
                table.PrimaryKey("PK_Students", x => x.Id);
            });
    }

    protected override void Down(MigrationBuilder migrationBuilder)
    {
        migrationBuilder.DropTable(name: "Students");
    }
}

Adding Foreign Key Migration

dotnet ef migrations add AddExamToStudent

Generated SQL:

protected override void Up(MigrationBuilder migrationBuilder)
{
    migrationBuilder.CreateTable(
        name: "Exams",
        columns: table => new
        {
            Id = table.Column<int>(nullable: false)
                .Annotation("SqlServer:Identity", "1, 1"),
            ExamName = table.Column<string>(maxLength: 100, nullable: false),
            SubjectId = table.Column<int>(nullable: false),
            ExamDate = table.Column<DateTime>(nullable: false)
        },
        constraints: table =>
        {
            table.PrimaryKey("PK_Exams", x => x.Id);
            table.ForeignKey(
                name: "FK_Exams_Subjects_SubjectId",
                column: x => x.SubjectId,
                principalTable: "Subjects",
                principalColumn: "Id",
                onDelete: ReferentialAction.Cascade);
        });
}

Updating Existing Migration (Before Apply)

If migration not yet applied:

# Remove the migration (before update)
dotnet ef migrations remove

# Modify your entities

# Create new migration
dotnet ef migrations add FixedMigrationName

# Apply
dotnet ef database update

Reverting Applied Migrations

Revert to previous migration:

# List migrations
dotnet ef migrations list

# Revert to specific migration (this runs Down method)
dotnet ef database update PreviousMigration

Generate SQL Script (Without Applying)

# Generate SQL to file
dotnet ef migrations script > migrations.sql

# Generate SQL from specific migration range
dotnet ef migrations script AddStudents AddFees > fee-migrations.sql

Best Practices

✓ Always review generated migrations before applying ✓ Never modify applied migrations (create new one instead) ✓ Use descriptive migration names ✓ Commit migrations to git ✓ Apply migrations in order

💡 Safe Schema Changes

For production:

1. Run migration in staging first
2. Review generated SQL
3. Backup database before applying
4. Apply with rollback plan ready

🤖Use AI to Learn Faster

Use ChatGPT, Claude, or Copilot to go deeper on Migrations. Try these prompts:

• "What's the difference between 'add migration' and 'update database'?"
• "How do you revert a migration that was already applied?"
• "Why should you review the generated SQL before applying?"
• "What's stored in the __EFMigrationsHistory table?"

💡 Tip: After reading this article, paste your own code into AI and ask "What could go wrong here and why?" — fastest way to find edge cases and deepen understanding.

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