What migrations are and why they exist

Beginner5 min read·lv-14-001

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Concept

Migrations are version-controlled database schema changes. Instead of manually altering the database or sharing SQL dump files, migrations are PHP files that describe schema changes in code — committed to the repository alongside application code.

Why migrations:

Version control: Schema changes are tracked in git, with history, authorship, and diffs.
Repeatability: Any developer can run php artisan migrate and get the same schema.
Environments: Dev, staging, and production schemas stay in sync.
Rollback: migrate:rollback undoes the last batch of migrations, enabling recovery from bad deploys.
Team collaboration: Multiple developers can add migrations independently; conflicts in migration files are explicit.

How migrations work:

Migration files live in database/migrations/, named YYYY_MM_DD_HHMMSS_description.php.
php artisan migrate reads all migrations not yet in the migrations table.
Runs each migration's up() method.
Records the migration name and batch number in the migrations table.
migrate:rollback finds the latest batch and runs each migration's down() method.

The migrations table: Created by php artisan migrate on first run. Stores migration (filename), batch (integer). All migrations in one migrate run share the same batch number.

Schema facade: Illuminate\Support\Facades\Schema. Used inside migrations to create/modify tables.

Code Example

php

<?php
// database/migrations/2024_01_15_120000_create_orders_table.php

use Illuminate\Database\Migrations\Migration;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\Schema;

return new class extends Migration
{
    // up() — apply the change
    public function up(): void
    {
        Schema::create('orders', function (Blueprint $table) {
            $table->id();                    // BIGINT UNSIGNED AUTO INCREMENT PRIMARY KEY
            $table->foreignId('user_id')
                  ->constrained()
                  ->onDelete('cascade');
            $table->decimal('total', 10, 2);
            $table->string('status', 20)->default('pending');
            $table->timestamps();            // created_at, updated_at
        });
    }

    // down() — undo the change (for rollback)
    public function down(): void
    {
        Schema::dropIfExists('orders');
    }
};

// Running migrations
// php artisan migrate                 → run all pending migrations
// php artisan migrate --force         → run in production (bypasses prompt)
// php artisan migrate:status          → show which migrations have run
// php artisan migrate:rollback        → roll back last batch
// php artisan migrate:rollback --step=3  → roll back 3 batches
// php artisan migrate:reset           → roll back ALL migrations
// php artisan migrate:fresh           → drop all tables + re-migrate (dev only!)
// php artisan migrate:fresh --seed    → fresh migrate + seed
// php artisan migrate:refresh         → rollback all + re-migrate

PreviousQuery Builder internals — how the fluent interface compiles to SQLQuery Builder NextCreating and running migrations — artisan make:migration, migrateMigrations & Schema