migrations readme

backend/migrations/README.md

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+# Database Migrations
+
+This project uses Sequelize CLI for database migrations. Migrations provide version control for your database schema.
+
+## Quick Reference
+
+```bash
+# Run pending migrations
+npm run db:migrate
+
+# Undo last migration
+npm run db:migrate:undo
+
+# Undo all migrations
+npm run db:migrate:undo:all
+
+# Check migration status
+npm run db:migrate:status
+
+# Test all migrations (up, down, up)
+npm run test:migrations
+```
+
+## Available Commands
+
+### `npm run db:migrate`
+
+**Purpose:** Runs all pending migrations that haven't been applied yet.
+
+- Checks the `SequelizeMeta` table to see which migrations have already run
+- Executes the `up` function in each pending migration file in order
+- Records each successful migration in the `SequelizeMeta` table
+- **When to use:** Deploy new schema changes to your database
+
+### `npm run db:migrate:undo`
+
+**Purpose:** Rolls back the most recent migration.
+
+- Executes the `down` function of the last applied migration
+- Removes that migration's entry from `SequelizeMeta`
+- **When to use:** Quickly revert the last change if something went wrong
+
+### `npm run db:migrate:undo:all`
+
+**Purpose:** Rolls back ALL migrations, returning to an empty database.
+
+- Executes the `down` function of every migration in reverse order
+- Clears the `SequelizeMeta` table
+- **When to use:** Reset development database to start fresh, or in testing
+
+### `npm run db:migrate:status`
+
+**Purpose:** Shows the status of all migrations.
+
+- Lists which migrations have been executed (with timestamps)
+- Shows which migrations are pending
+- **When to use:** Check what's been applied before deploying or debugging
+
+### `npm run db:create`
+
+**Purpose:** Creates the database specified in your config.
+
+- Reads `DB_NAME` from environment variables
+- Creates a new PostgreSQL database with that name
+- **When to use:** Initial setup on a new environment
+
+### `npm run test:migrations`
+
+**Purpose:** Automated testing of migrations (to be implemented in Phase 4).
+
+- Will create a fresh test database
+- Run all migrations up, then down, then up again
+- Verify migrations are reversible and idempotent
+- **When to use:** In CI/CD pipeline before merging migration changes
+
+## Environment Configuration
+
+All commands use the environment specified by `NODE_ENV` (dev, test, qa, prod) and load the corresponding `.env` file automatically.
+
+Examples:
+
+```bash
+# Run migrations in development
+NODE_ENV=dev npm run db:migrate
+
+# Check status in QA environment
+NODE_ENV=qa npm run db:migrate:status
+
+# Run migrations in production
+NODE_ENV=prod npm run db:migrate
+```
+
+## Creating a New Migration
+
+```bash
+# Generate a new migration file
+npx sequelize-cli migration:generate --name description-of-change
+```
+
+This creates a timestamped file in `backend/migrations/`:
+
+```
+20241125123456-description-of-change.js
+```
+
+### Migration File Structure
+
+```javascript
+module.exports = {
+  up: async (queryInterface, Sequelize) => {
+    // Schema changes to apply
+  },
+  down: async (queryInterface, Sequelize) => {
+    // How to revert the changes
+  },
+};
+```
+
+## Naming Conventions
+
+Use descriptive names that indicate the action:
+
+- `create-users` - Creating a new table
+- `add-email-to-users` - Adding a column
+- `remove-legacy-field-from-items` - Removing a column
+- `add-index-on-users-email` - Adding an index
+- `change-status-enum-in-rentals` - Modifying a column
+
+## Zero-Downtime Patterns
+
+### Adding a Column (Safe)
+
+```javascript
+up: async (queryInterface, Sequelize) => {
+  await queryInterface.addColumn("users", "newField", {
+    type: Sequelize.STRING,
+    allowNull: true, // Must be nullable or have default
+  });
+};
+```
+
+### Adding a NOT NULL Column
+
+```javascript
+// Step 1: Add as nullable
+up: async (queryInterface, Sequelize) => {
+  await queryInterface.addColumn("users", "newField", {
+    type: Sequelize.STRING,
+    allowNull: true,
+  });
+};
+
+// Step 2: Backfill data (separate migration)
+up: async (queryInterface, Sequelize) => {
+  await queryInterface.sequelize.query(
+    `UPDATE users SET "newField" = 'default_value' WHERE "newField" IS NULL`
+  );
+};
+
+// Step 3: Add NOT NULL constraint (separate migration, after code deployed)
+up: async (queryInterface, Sequelize) => {
+  await queryInterface.changeColumn("users", "newField", {
+    type: Sequelize.STRING,
+    allowNull: false,
+  });
+};
+```
+
+### Removing a Column (3-Step Process)
+
+1. **Deploy 1**: Update code to stop reading/writing the column
+2. **Deploy 2**: Run migration to remove column
+3. **Deploy 3**: Remove column references from model (cleanup)
+
+```javascript
+// Migration in Deploy 2
+up: async (queryInterface) => {
+  await queryInterface.removeColumn('users', 'oldField');
+},
+down: async (queryInterface, Sequelize) => {
+  await queryInterface.addColumn('users', 'oldField', {
+    type: Sequelize.STRING,
+    allowNull: true
+  });
+}
+```
+
+### Renaming a Column (3-Step Process)
+
+```javascript
+// Deploy 1: Add new column, copy data
+up: async (queryInterface, Sequelize) => {
+  await queryInterface.addColumn("users", "newName", {
+    type: Sequelize.STRING,
+  });
+  await queryInterface.sequelize.query(
+    'UPDATE users SET "newName" = "oldName"'
+  );
+};
+
+// Deploy 2: Update code to use newName, keep oldName as fallback
+// (no migration)
+
+// Deploy 3: Remove old column
+up: async (queryInterface) => {
+  await queryInterface.removeColumn("users", "oldName");
+};
+```
+
+### Creating Indexes (Use CONCURRENTLY)
+
+```javascript
+up: async (queryInterface) => {
+  await queryInterface.addIndex("users", ["email"], {
+    unique: true,
+    concurrently: true, // Prevents table locking
+    name: "users_email_unique",
+  });
+};
+```
+
+## Testing Checklist
+
+Before committing a migration:
+
+- [ ] Migration has both `up` and `down` functions
+- [ ] Tested locally: `npm run db:migrate`
+- [ ] Tested rollback: `npm run db:migrate:undo`
+- [ ] Tested re-apply: `npm run db:migrate`
+- [ ] Full test: `npm run test:migrations`
+- [ ] Application starts and works correctly
+- [ ] No data loss in `down` migration (where possible)
+
+## Deployment Checklist
+
+Before deploying to production:
+
+- [ ] Backup production database
+- [ ] Test migration on copy of production data
+- [ ] Review migration for safety (no destructive operations)
+- [ ] Schedule during low-traffic window
+- [ ] Have rollback plan ready
+- [ ] Monitor logs after deployment
+
+## Rollback Procedures
+
+### Undo Last Migration
+
+```bash
+npm run db:migrate:undo
+```
+
+### Undo Multiple Migrations
+
+```bash
+# Undo last 3 migrations
+npx sequelize-cli db:migrate:undo --step 3
+```
+
+### Undo to Specific Migration
+
+```bash
+npx sequelize-cli db:migrate:undo:all --to 20241124000005-create-rentals.js
+```
+
+## Common Issues
+
+### "Migration file not found"
+
+Ensure the migration filename in `SequelizeMeta` matches the file on disk.
+
+### "Column already exists"
+
+The migration may have partially run. Check the schema and either:
+
+- Manually fix the schema
+- Mark migration as complete: `INSERT INTO "SequelizeMeta" VALUES ('filename.js')`
+
+### "Cannot drop column - dependent objects"
+
+Drop dependent indexes/constraints first:
+
+```javascript
+await queryInterface.removeIndex("users", "index_name");
+await queryInterface.removeColumn("users", "column_name");
+```
+
+### Foreign Key Constraint Failures
+
+Ensure data integrity before adding constraints:
+
+```javascript
+// Clean orphaned records first
+await queryInterface.sequelize.query(
+  'DELETE FROM rentals WHERE "itemId" NOT IN (SELECT id FROM items)'
+);
+// Then add constraint
+await queryInterface.addConstraint("rentals", {
+  fields: ["itemId"],
+  type: "foreign key",
+  references: { table: "items", field: "id" },
+});
+```