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+# Database Adapter Implementation Guide
+
+Instructions for implementing or modifying database adapters in DatabaseKit.
+
+---
+
+## 🎯 Adapter Purpose
+
+Adapters translate the unified `Repository<T>` interface to database-specific operations.
+Each adapter encapsulates ALL database-specific logic.
+
+---
+
+## 📐 Adapter Structure
+
+Every adapter MUST implement these components:
+
+```typescript
+class DatabaseAdapter {
+  // 1. Connection Management
+  connect(uri: string, options?: ConnectionOptions): Promise<void>;
+  disconnect(): Promise<void>;
+  isConnected(): boolean;
+
+  // 2. Repository Factory
+  createRepository<T>(options: RepositoryOptions<T>): Repository<T>;
+
+  // 3. Transaction Support
+  withTransaction<R>(fn: (ctx: TransactionContext) => Promise<R>): Promise<R>;
+
+  // 4. Health Check
+  healthCheck(): Promise<HealthCheckResult>;
+}
+```
+
+---
+
+## ✅ Implementation Checklist
+
+When creating or modifying an adapter:
+
+### Connection
+
+- [ ] Implement connection with retry logic
+- [ ] Handle connection pooling via `PoolConfig`
+- [ ] Implement graceful disconnect
+- [ ] Add connection state tracking
+
+### Repository Methods (ALL required)
+
+- [ ] `create(data)` - Insert single document
+- [ ] `findById(id)` - Find by primary key
+- [ ] `findOne(filter)` - Find first match
+- [ ] `findAll(filter)` - Find all matches
+- [ ] `findPage(options)` - Paginated query
+- [ ] `updateById(id, data)` - Update by primary key
+- [ ] `deleteById(id)` - Delete by primary key
+- [ ] `count(filter)` - Count matches
+- [ ] `exists(filter)` - Check existence
+- [ ] `insertMany(data[])` - Bulk insert
+- [ ] `updateMany(filter, data)` - Bulk update
+- [ ] `deleteMany(filter)` - Bulk delete
+- [ ] `upsert(filter, data)` - Update or insert
+- [ ] `distinct(field, filter)` - Unique values
+- [ ] `select(filter, fields)` - Field projection
+
+### Optional Features
+
+- [ ] `softDelete(id)` - When `softDelete: true`
+- [ ] `restore(id)` - When `softDelete: true`
+- [ ] `findWithDeleted(filter)` - When `softDelete: true`
+
+### Cross-Cutting
+
+- [ ] Timestamps (`createdAt`, `updatedAt`)
+- [ ] Hooks (`beforeCreate`, `afterCreate`, etc.)
+- [ ] Transaction context repositories
+- [ ] Health check with connection details
+
+---
+
+## 🔧 MongoDB Adapter Patterns
+
+### Model Registration
+
+```typescript
+// Use Mongoose schema
+const schema = new Schema<T>(definition, { timestamps: true });
+const model = mongoose.model<T>(name, schema);
+```
+
+### Query Translation
+
+```typescript
+// MongoDB operators are native
+{
+  age: {
+    $gte: 18;
+  }
+} // Direct pass-through
+{
+  status: {
+    $in: ["active", "pending"];
+  }
+}
+```
+
+### Transactions
+
+```typescript
+// Use ClientSession
+const session = await mongoose.startSession();
+session.startTransaction();
+try {
+  await model.create([data], { session });
+  await session.commitTransaction();
+} catch (e) {
+  await session.abortTransaction();
+  throw e;
+} finally {
+  session.endSession();
+}
+```
+
+### ID Handling
+
+```typescript
+// MongoDB uses ObjectId
+import { Types } from "mongoose";
+const objectId = new Types.ObjectId(id);
+```
+
+---
+
+## 🔧 PostgreSQL Adapter Patterns
+
+### Table Configuration
+
+```typescript
+// Use Knex table name
+const table = knex<T>(tableName);
+```
+
+### Query Translation
+
+```typescript
+// Convert operators to SQL
+{ price: { gt: 100 } }       → .where('price', '>', 100)
+{ status: { in: [...] } }    → .whereIn('status', [...])
+{ name: { like: '%john%' } } → .whereILike('name', '%john%')
+{ deleted: { isNull: true }} → .whereNull('deleted')
+```
+
+### Transactions
+
+```typescript
+// Use Knex transaction
+await knex.transaction(async (trx) => {
+  await trx("users").insert(data);
+  await trx("orders").insert(orderData);
+});
+```
+
+### ID Handling
+
+```typescript
+// PostgreSQL uses auto-increment or UUID
+// Return inserted row to get ID
+const [inserted] = await knex("users").insert(data).returning("*");
+```
+
+---
+
+## 🪝 Hook Implementation
+
+All adapters must support lifecycle hooks:
+
+```typescript
+interface RepositoryHooks<T> {
+  beforeCreate?(ctx: HookContext<T>): Partial<T> | Promise<Partial<T>>;
+  afterCreate?(entity: T): void | Promise<void>;
+  beforeUpdate?(ctx: HookContext<T>): Partial<T> | Promise<Partial<T>>;
+  afterUpdate?(entity: T | null): void | Promise<void>;
+  beforeDelete?(id: string | number): void | Promise<void>;
+  afterDelete?(success: boolean): void | Promise<void>;
+}
+```
+
+### Hook Execution Order
+
+1. `beforeCreate` → modify data → `create()` → `afterCreate`
+2. `beforeUpdate` → modify data → `updateById()` → `afterUpdate`
+3. `beforeDelete` → `deleteById()` → `afterDelete`
+
+### Hook Context
+
+```typescript
+interface HookContext<T> {
+  data: Partial<T>; // The data being created/updated
+  repository: Repository<T>; // Self-reference for lookups
+}
+```
+
+---
+
+## ⏱️ Timestamp Implementation
+
+When `timestamps: true`:
+
+```typescript
+// On create
+data.createdAt = new Date();
+data.updatedAt = new Date();
+
+// On update
+data.updatedAt = new Date();
+// Never modify createdAt on update!
+```
+
+---
+
+## 🗑️ Soft Delete Implementation
+
+When `softDelete: true`:
+
+```typescript
+// softDelete() - Set deletedAt
+await repo.updateById(id, { deletedAt: new Date() });
+
+// restore() - Clear deletedAt
+await repo.updateById(id, { deletedAt: null });
+
+// findAll/findOne - Exclude deleted
+filter.deletedAt = { isNull: true }; // or { $eq: null } for Mongo
+
+// findWithDeleted - Include deleted
+// Skip the deletedAt filter
+```
+
+---
+
+## 🧪 Testing Requirements
+
+Every adapter must have tests for:
+
+1. **Connection** - Connect, disconnect, reconnect
+2. **CRUD** - All basic operations
+3. **Bulk** - insertMany, updateMany, deleteMany
+4. **Queries** - Filters, pagination, sorting
+5. **Transactions** - Commit, rollback, nested
+6. **Hooks** - All 6 hooks fire correctly
+7. **Timestamps** - createdAt/updatedAt auto-set
+8. **Soft Delete** - delete, restore, findWithDeleted
+9. **Health Check** - Returns correct status
+
+---
+
+## 📝 Adding a New Adapter
+
+To add a new database (e.g., SQLite, MySQL):
+
+1. Create `src/adapters/sqlite.adapter.ts`
+2. Implement `DatabaseAdapter` interface
+3. Add to `DatabaseService`:
+   ```typescript
+   createSqliteRepository<T>(opts): Repository<T>
+   ```
+4. Add connection config type to `DatabaseConfig`
+5. Add comprehensive tests
+6. Export from `index.ts` if public
+7. Document in README
+
+---
+
+_Last updated: February 2026_