diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 75cadcd..a8180aa 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -25,6 +25,15 @@ jobs:
           --health-interval 10s
           --health-timeout 5s
           --health-retries 20
+      mongo:
+        image: mongo:6
+        ports:
+          - 27017:27017
+        options: >-
+          --health-cmd "mongosh --eval 'db.runCommand({ ping: 1 })' || exit 1"
+          --health-interval 10s
+          --health-timeout 5s
+          --health-retries 30
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -33,7 +42,7 @@ jobs:
         uses: shivammathur/setup-php@v2
         with:
           php-version: ${{ matrix.php }}
-          extensions: pdo, pdo_mysql, pdo_sqlite
+          extensions: pdo, pdo_mysql, pdo_sqlite, mongodb
           coverage: none
 
       - name: Install dependencies
@@ -59,5 +68,7 @@ jobs:
           MYSQL_DB: pairity
           MYSQL_USER: root
           MYSQL_PASS: root
+          MONGO_HOST: 127.0.0.1
+          MONGO_PORT: 27017
         run: |
           vendor/bin/phpunit --colors=always
diff --git a/README.md b/README.md
index f881012..0ad7e07 100644
--- a/README.md
+++ b/README.md
@@ -129,7 +129,52 @@ Notes:
 - SQL Server
 - Oracle
 
-NoSQL: a minimal in‑memory MongoDB stub is included (`Pairity\NoSql\Mongo\MongoConnectionInterface` and `MongoConnection`) for experimentation without external deps.
+NoSQL:
+- MongoDB (production): `Pairity\NoSql\Mongo\MongoClientConnection` via `mongodb/mongodb` + `ext-mongodb`.
+- MongoDB (stub): `Pairity\NoSql\Mongo\MongoConnection` (in‑memory) remains for experimentation without external deps.
+
+### MongoDB (production adapter)
+
+Pairity includes a production‑ready MongoDB adapter that wraps the official `mongodb/mongodb` library.
+
+Requirements:
+- PHP `ext-mongodb` (installed in PHP), and Composer dependency `mongodb/mongodb` (already required by this package).
+
+Connect using the `MongoConnectionManager`:
+
+```php
+use Pairity\NoSql\Mongo\MongoConnectionManager;
+
+// Option A: Full URI
+$conn = MongoConnectionManager::make([
+    'uri' => 'mongodb://user:pass@127.0.0.1:27017/?authSource=admin',
+]);
+
+// Option B: Discrete params
+$conn = MongoConnectionManager::make([
+    'host' => '127.0.0.1',
+    'port' => 27017,
+    // 'username' => 'user',
+    // 'password' => 'pass',
+    // 'authSource' => 'admin',
+    // 'replicaSet' => 'rs0',
+    // 'tls' => false,
+]);
+
+// Basic CRUD
+$db = 'app';
+$col = 'users';
+
+$id = $conn->insertOne($db, $col, ['email' => 'mongo@example.com', 'name' => 'Alice']);
+$one = $conn->find($db, $col, ['_id' => $id]);
+$conn->updateOne($db, $col, ['_id' => $id], ['$set' => ['name' => 'Alice Updated']]);
+$conn->deleteOne($db, $col, ['_id' => $id]);
+```
+
+Notes:
+- `_id` strings that look like 24‑hex ObjectIds are automatically converted to `ObjectId` on input; returned documents convert `ObjectId` back to strings.
+- Aggregation pipelines are supported via `$conn->aggregate($db, $collection, $pipeline, $options)`.
+- See `examples/nosql/mongo_crud.php` for a runnable demo.
 
 ## Raw SQL
 
diff --git a/bin/pairity b/bin/pairity
index 9266803..561ce65 100644
--- a/bin/pairity
+++ b/bin/pairity
@@ -133,6 +133,9 @@ Usage:
   pairity status [--path=DIR] [--config=FILE]
   pairity reset [--config=FILE]
   pairity make:migration Name [--path=DIR]
+  pairity mongo:index:ensure DB COLLECTION KEYS_JSON [--unique]
+  pairity mongo:index:drop DB COLLECTION NAME
+  pairity mongo:index:list DB COLLECTION
 
 Environment:
   DB_DRIVER, DB_HOST, DB_PORT, DB_DATABASE, DB_USERNAME, DB_PASSWORD, DB_PATH (for sqlite)
@@ -243,6 +246,51 @@ PHP;
             stdout('Created: ' . $file);
             break;
 
+        case 'mongo:index:ensure':
+            // Args: DB COLLECTION KEYS_JSON [--unique]
+            $db = $args[0] ?? null;
+            $col = $args[1] ?? null;
+            $keysJson = $args[2] ?? null;
+            if (!$db || !$col || !$keysJson) {
+                stderr('Usage: pairity mongo:index:ensure DB COLLECTION KEYS_JSON [--unique]');
+                exit(1);
+            }
+            $config = loadConfig($args);
+            $conn = \Pairity\NoSql\Mongo\MongoConnectionManager::make($config);
+            $idx = new \Pairity\NoSql\Mongo\IndexManager($conn, $db, $col);
+            $keys = json_decode($keysJson, true);
+            if (!is_array($keys)) { stderr('Invalid KEYS_JSON (must be object like {"email":1})'); exit(1); }
+            $opts = [];
+            if (!empty($args['unique'])) { $opts['unique'] = true; }
+            $name = $idx->ensureIndex($keys, $opts);
+            stdout('Ensured index: ' . $name);
+            break;
+
+        case 'mongo:index:drop':
+            // Args: DB COLLECTION NAME
+            $db = $args[0] ?? null;
+            $col = $args[1] ?? null;
+            $name = $args[2] ?? null;
+            if (!$db || !$col || !$name) { stderr('Usage: pairity mongo:index:drop DB COLLECTION NAME'); exit(1); }
+            $config = loadConfig($args);
+            $conn = \Pairity\NoSql\Mongo\MongoConnectionManager::make($config);
+            $idx = new \Pairity\NoSql\Mongo\IndexManager($conn, $db, $col);
+            $idx->dropIndex($name);
+            stdout('Dropped index: ' . $name);
+            break;
+
+        case 'mongo:index:list':
+            // Args: DB COLLECTION
+            $db = $args[0] ?? null;
+            $col = $args[1] ?? null;
+            if (!$db || !$col) { stderr('Usage: pairity mongo:index:list DB COLLECTION'); exit(1); }
+            $config = loadConfig($args);
+            $conn = \Pairity\NoSql\Mongo\MongoConnectionManager::make($config);
+            $idx = new \Pairity\NoSql\Mongo\IndexManager($conn, $db, $col);
+            $list = $idx->listIndexes();
+            stdout(json_encode($list));
+            break;
+
         default:
             cmd_help();
             break;
diff --git a/composer.json b/composer.json
index 2d37d3e..c7e1d3d 100644
--- a/composer.json
+++ b/composer.json
@@ -7,7 +7,9 @@
     { "name": "Pairity Contributors" }
   ],
   "require": {
-    "php": ">=8.1"
+    "php": ">=8.1",
+    "ext-mongodb": "*",
+    "mongodb/mongodb": "^1.19"
   },
   "autoload": {
     "psr-4": {
diff --git a/examples/nosql/mongo_crud.php b/examples/nosql/mongo_crud.php
new file mode 100644
index 0000000..201a0eb
--- /dev/null
+++ b/examples/nosql/mongo_crud.php
@@ -0,0 +1,50 @@
+<?php
+
+declare(strict_types=1);
+
+require __DIR__ . '/../../vendor/autoload.php';
+
+use Pairity\NoSql\Mongo\MongoConnectionManager;
+
+// Configure via URI or discrete params
+$conn = MongoConnectionManager::make([
+    // 'uri' => 'mongodb://user:pass@127.0.0.1:27017/?authSource=admin',
+    'host' => '127.0.0.1',
+    'port' => 27017,
+]);
+
+$db = 'pairity_demo';
+$col = 'users';
+
+// Clean collection for demo
+foreach ($conn->find($db, $col, []) as $doc) {
+    $conn->deleteOne($db, $col, ['_id' => $doc['_id']]);
+}
+
+// Insert
+$id = $conn->insertOne($db, $col, [
+    'email' => 'mongo@example.com',
+    'name'  => 'Mongo User',
+    'status'=> 'active',
+]);
+echo "Inserted _id={$id}\n";
+
+// Find
+$found = $conn->find($db, $col, ['_id' => $id]);
+echo 'Found: ' . json_encode($found, JSON_UNESCAPED_SLASHES) . PHP_EOL;
+
+// Update
+$conn->updateOne($db, $col, ['_id' => $id], ['$set' => ['name' => 'Updated Mongo User']]);
+$after = $conn->find($db, $col, ['_id' => $id]);
+echo 'After update: ' . json_encode($after, JSON_UNESCAPED_SLASHES) . PHP_EOL;
+
+// Aggregate (simple match projection)
+$agg = $conn->aggregate($db, $col, [
+    ['$match' => ['status' => 'active']],
+    ['$project' => ['email' => 1, 'name' => 1]],
+]);
+echo 'Aggregate: ' . json_encode($agg, JSON_UNESCAPED_SLASHES) . PHP_EOL;
+
+// Delete
+$deleted = $conn->deleteOne($db, $col, ['_id' => $id]);
+echo "Deleted: {$deleted}\n";
diff --git a/examples/nosql/mongo_dao_crud.php b/examples/nosql/mongo_dao_crud.php
new file mode 100644
index 0000000..afdbd0f
--- /dev/null
+++ b/examples/nosql/mongo_dao_crud.php
@@ -0,0 +1,62 @@
+<?php
+
+declare(strict_types=1);
+
+require __DIR__ . '/../../vendor/autoload.php';
+
+use Pairity\NoSql\Mongo\MongoConnectionManager;
+use Pairity\NoSql\Mongo\AbstractMongoDao;
+use Pairity\Model\AbstractDto;
+
+// Connect via URI or discrete params
+$conn = MongoConnectionManager::make([
+    // 'uri' => 'mongodb://user:pass@127.0.0.1:27017/?authSource=admin',
+    'host' => '127.0.0.1',
+    'port' => 27017,
+]);
+
+class UserDoc extends AbstractDto {}
+
+class UserMongoDao extends AbstractMongoDao
+{
+    protected function collection(): string { return 'pairity_demo.users'; }
+    protected function dtoClass(): string { return UserDoc::class; }
+}
+
+$dao = new UserMongoDao($conn);
+
+// Clean for demo
+foreach ($dao->findAllBy([]) as $dto) {
+    $id = (string)($dto->toArray(false)['_id'] ?? '');
+    if ($id) { $dao->deleteById($id); }
+}
+
+// Insert
+$created = $dao->insert([
+    'email' => 'mongo@example.com',
+    'name'  => 'Mongo User',
+    'status'=> 'active',
+]);
+echo 'Inserted: ' . json_encode($created->toArray(false)) . "\n";
+
+// Find by dynamic helper
+$found = $dao->findOneByEmail('mongo@example.com');
+echo 'Found: ' . json_encode($found?->toArray(false)) . "\n";
+
+// Update
+if ($found) {
+    $id = (string)$found->toArray(false)['_id'];
+    $updated = $dao->update($id, ['name' => 'Updated Mongo User']);
+    echo 'Updated: ' . json_encode($updated->toArray(false)) . "\n";
+}
+
+// Projection + sort + limit
+$list = $dao->fields('email', 'name')->sort(['email' => 1])->limit(10)->findAllBy(['status' => 'active']);
+echo 'List (projected): ' . json_encode(array_map(fn($d) => $d->toArray(false), $list)) . "\n";
+
+// Delete
+if ($found) {
+    $id = (string)$found->toArray(false)['_id'];
+    $deleted = $dao->deleteById($id);
+    echo "Deleted: {$deleted}\n";
+}
diff --git a/examples/nosql/mongo_relations_demo.php b/examples/nosql/mongo_relations_demo.php
new file mode 100644
index 0000000..439aa54
--- /dev/null
+++ b/examples/nosql/mongo_relations_demo.php
@@ -0,0 +1,75 @@
+<?php
+
+declare(strict_types=1);
+
+require __DIR__ . '/../../vendor/autoload.php';
+
+use Pairity\NoSql\Mongo\MongoConnectionManager;
+use Pairity\NoSql\Mongo\AbstractMongoDao;
+use Pairity\Model\AbstractDto;
+
+class UserDoc extends AbstractDto {}
+class PostDoc extends AbstractDto {}
+
+class UserMongoDao extends AbstractMongoDao
+{
+    protected function collection(): string { return 'pairity_demo.users'; }
+    protected function dtoClass(): string { return UserDoc::class; }
+    protected function relations(): array
+    {
+        return [
+            'posts' => [
+                'type' => 'hasMany',
+                'dao'  => PostMongoDao::class,
+                'foreignKey' => 'user_id',
+                'localKey'   => '_id',
+            ],
+        ];
+    }
+}
+
+class PostMongoDao extends AbstractMongoDao
+{
+    protected function collection(): string { return 'pairity_demo.posts'; }
+    protected function dtoClass(): string { return PostDoc::class; }
+    protected function relations(): array
+    {
+        return [
+            'user' => [
+                'type' => 'belongsTo',
+                'dao'  => UserMongoDao::class,
+                'foreignKey' => 'user_id',
+                'otherKey'   => '_id',
+            ],
+        ];
+    }
+}
+
+$conn = MongoConnectionManager::make([
+    'host' => '127.0.0.1',
+    'port' => 27017,
+]);
+
+$userDao = new UserMongoDao($conn);
+$postDao = new PostMongoDao($conn);
+
+// Clean
+foreach ($postDao->findAllBy([]) as $p) { $postDao->deleteById((string)$p->toArray(false)['_id']); }
+foreach ($userDao->findAllBy([]) as $u) { $userDao->deleteById((string)$u->toArray(false)['_id']); }
+
+// Seed
+$u = $userDao->insert(['email' => 'mongo@example.com', 'name' => 'Alice']);
+$uid = (string)$u->toArray(false)['_id'];
+$p1 = $postDao->insert(['title' => 'First', 'user_id' => $uid]);
+$p2 = $postDao->insert(['title' => 'Second', 'user_id' => $uid]);
+
+// Eager load posts on users
+$users = $userDao->fields('email', 'name', 'posts.title')->with(['posts'])->findAllBy([]);
+echo 'Users with posts: ' . json_encode(array_map(fn($d) => $d->toArray(), $users)) . "\n";
+
+// Lazy load user on a post
+$onePost = $postDao->findOneBy(['title' => 'First']);
+if ($onePost) {
+    $postDao->load($onePost, 'user');
+    echo 'Post with user: ' . json_encode($onePost->toArray()) . "\n";
+}
diff --git a/src/NoSql/Mongo/AbstractMongoDao.php b/src/NoSql/Mongo/AbstractMongoDao.php
new file mode 100644
index 0000000..111f559
--- /dev/null
+++ b/src/NoSql/Mongo/AbstractMongoDao.php
@@ -0,0 +1,386 @@
+<?php
+
+namespace Pairity\NoSql\Mongo;
+
+use Pairity\Model\AbstractDto;
+
+/**
+ * Base DAO for MongoDB collections returning DTOs.
+ *
+ * Usage: extend and implement collection() + dtoClass().
+ */
+abstract class AbstractMongoDao
+{
+    protected MongoConnectionInterface $connection;
+
+    /** @var array<int,string>|null */
+    private ?array $projection = null; // list of field names to include
+    /** @var array<string,int> */
+    private array $sortSpec = [];
+    private ?int $limitVal = null;
+    private ?int $skipVal = null;
+
+    /** @var array<int,string> */
+    private array $with = [];
+    /** @var array<string, array<int,string>> */
+    private array $relationFields = [];
+
+    public function __construct(MongoConnectionInterface $connection)
+    {
+        $this->connection = $connection;
+    }
+
+    /** Collection name (e.g., "users"). */
+    abstract protected function collection(): string;
+
+    /** @return class-string<AbstractDto> */
+    abstract protected function dtoClass(): string;
+
+    /** Access to underlying connection. */
+    public function getConnection(): MongoConnectionInterface
+    {
+        return $this->connection;
+    }
+
+    /** Relation metadata (MVP). Override in concrete DAO. */
+    protected function relations(): array
+    {
+        return [];
+    }
+
+    // ========= Query modifiers =========
+
+    /**
+     * Specify projection fields to include on base entity and optionally on relations via dot-notation.
+     * Example: fields('email','name','posts.title')
+     */
+    public function fields(string ...$fields): static
+    {
+        $base = [];
+        foreach ($fields as $f) {
+            $f = (string)$f;
+            if ($f === '') continue;
+            if (str_contains($f, '.')) {
+                [$rel, $col] = explode('.', $f, 2);
+                if ($rel !== '' && $col !== '') {
+                    $this->relationFields[$rel][] = $col;
+                }
+            } else {
+                $base[] = $f;
+            }
+        }
+        $this->projection = $base ?: null;
+        return $this;
+    }
+
+    /** Sorting spec, e.g., sort(['created_at' => -1]) */
+    public function sort(array $spec): static
+    {
+        // sanitize values to 1 or -1
+        $out = [];
+        foreach ($spec as $k => $v) {
+            $out[(string)$k] = ((int)$v) < 0 ? -1 : 1;
+        }
+        $this->sortSpec = $out;
+        return $this;
+    }
+
+    public function limit(int $n): static
+    {
+        $this->limitVal = max(0, $n);
+        return $this;
+    }
+
+    public function skip(int $n): static
+    {
+        $this->skipVal = max(0, $n);
+        return $this;
+    }
+
+    // ========= CRUD =========
+
+    /** @param array<string,mixed>|Filter $filter */
+    public function findOneBy(array|Filter $filter): ?AbstractDto
+    {
+        $opts = $this->buildOptions();
+        $opts['limit'] = 1;
+        $docs = $this->connection->find($this->databaseName(), $this->collection(), $this->normalizeFilterInput($filter), $opts);
+        $this->resetModifiers();
+        $row = $docs[0] ?? null;
+        return $row ? $this->hydrate($row) : null;
+    }
+
+    /**
+     * @param array<string,mixed>|Filter $filter
+     * @param array<string,mixed> $options Additional options (merged after internal modifiers)
+     * @return array<int,AbstractDto>
+     */
+    public function findAllBy(array|Filter $filter = [], array $options = []): array
+    {
+        $opts = $this->buildOptions();
+        // external override/merge
+        foreach ($options as $k => $v) { $opts[$k] = $v; }
+        $docs = $this->connection->find($this->databaseName(), $this->collection(), $this->normalizeFilterInput($filter), $opts);
+        $dtos = array_map(fn($d) => $this->hydrate($d), is_iterable($docs) ? $docs : []);
+        if ($dtos && $this->with) {
+            $this->attachRelations($dtos);
+        }
+        $this->resetModifiers();
+        return $dtos;
+    }
+
+    public function findById(string $id): ?AbstractDto
+    {
+        return $this->findOneBy(['_id' => $id]);
+    }
+
+    /** @param array<string,mixed> $data */
+    public function insert(array $data): AbstractDto
+    {
+        $id = $this->connection->insertOne($this->databaseName(), $this->collection(), $data);
+        // fetch back
+        return $this->findById($id) ?? $this->hydrate(array_merge($data, ['_id' => $id]));
+    }
+
+    /** @param array<string,mixed> $data */
+    public function update(string $id, array $data): AbstractDto
+    {
+        $this->connection->updateOne($this->databaseName(), $this->collection(), ['_id' => $id], ['$set' => $data]);
+        return $this->findById($id) ?? $this->hydrate(array_merge($data, ['_id' => $id]));
+    }
+
+    public function deleteById(string $id): int
+    {
+        return $this->connection->deleteOne($this->databaseName(), $this->collection(), ['_id' => $id]);
+    }
+
+    /** @param array<string,mixed>|Filter $filter */
+    public function deleteBy(array|Filter $filter): int
+    {
+        // For MVP provide deleteOne semantic; bulk deletes could be added later
+        return $this->connection->deleteOne($this->databaseName(), $this->collection(), $this->normalizeFilterInput($filter));
+    }
+
+    /** Upsert by id convenience. */
+    public function upsertById(string $id, array $data): string
+    {
+        return $this->connection->upsertOne($this->databaseName(), $this->collection(), ['_id' => $id], ['$set' => $data]);
+    }
+
+    /** @param array<string,mixed>|Filter $filter @param array<string,mixed> $update */
+    public function upsertBy(array|Filter $filter, array $update): string
+    {
+        return $this->connection->upsertOne($this->databaseName(), $this->collection(), $this->normalizeFilterInput($filter), $update);
+    }
+
+    /**
+     * Fetch related docs where a field is within the given set of values.
+     * @param string $field
+     * @param array<int,string> $values
+     * @return array<int,AbstractDto>
+     */
+    public function findAllWhereIn(string $field, array $values): array
+    {
+        if (!$values) return [];
+        // Normalize values (unique)
+        $values = array_values(array_unique($values));
+        $opts = $this->buildOptions();
+        $docs = $this->connection->find($this->databaseName(), $this->collection(), [ $field => ['$in' => $values] ], $opts);
+        return array_map(fn($d) => $this->hydrate($d), is_iterable($docs) ? $docs : []);
+    }
+
+    // ========= Dynamic helpers =========
+
+    public function __call(string $name, array $arguments): mixed
+    {
+        if (preg_match('/^(findOneBy|findAllBy|updateBy|deleteBy)([A-Z][A-Za-z0-9_]*)$/', $name, $m)) {
+            $op = $m[1];
+            $col = $this->normalizeColumn($m[2]);
+            switch ($op) {
+                case 'findOneBy':
+                    return $this->findOneBy([$col => $arguments[0] ?? null]);
+                case 'findAllBy':
+                    return $this->findAllBy([$col => $arguments[0] ?? null]);
+                case 'updateBy':
+                    $value = $arguments[0] ?? null;
+                    $data = $arguments[1] ?? [];
+                    if (!is_array($data)) {
+                        throw new \InvalidArgumentException('updateBy* expects second argument as array $data');
+                    }
+                    $one = $this->findOneBy([$col => $value]);
+                    if (!$one) { return 0; }
+                    $id = (string)($one->toArray(false)['_id'] ?? '');
+                    $this->update($id, $data);
+                    return 1;
+                case 'deleteBy':
+                    return $this->deleteBy([$col => $arguments[0] ?? null]);
+            }
+        }
+        throw new \BadMethodCallException(static::class . "::{$name} does not exist");
+    }
+
+    // ========= Internals =========
+
+    protected function normalizeColumn(string $studly): string
+    {
+        $snake = preg_replace('/(?<!^)[A-Z]/', '_$0', $studly) ?? $studly;
+        return strtolower($snake);
+    }
+
+    protected function hydrate(array $doc): AbstractDto
+    {
+        // Ensure _id is a string for DTO friendliness
+        if (isset($doc['_id']) && !is_string($doc['_id'])) {
+            $doc['_id'] = (string)$doc['_id'];
+        }
+        $class = $this->dtoClass();
+        /** @var AbstractDto $dto */
+        $dto = $class::fromArray($doc);
+        return $dto;
+    }
+
+    /** @param array<string,mixed>|Filter $filter */
+    private function normalizeFilterInput(array|Filter $filter): array
+    {
+        if ($filter instanceof Filter) {
+            return $filter->toArray();
+        }
+        return $filter;
+    }
+
+    /** Build MongoDB driver options from current modifiers. */
+    private function buildOptions(): array
+    {
+        $opts = [];
+        if ($this->projection) {
+            $proj = [];
+            foreach ($this->projection as $f) { $proj[$f] = 1; }
+            $opts['projection'] = $proj;
+        }
+        if ($this->sortSpec) { $opts['sort'] = $this->sortSpec; }
+        if ($this->limitVal !== null) { $opts['limit'] = $this->limitVal; }
+        if ($this->skipVal !== null) { $opts['skip'] = $this->skipVal; }
+        return $opts;
+    }
+
+    private function resetModifiers(): void
+    {
+        $this->projection = null;
+        $this->sortSpec = [];
+        $this->limitVal = null;
+        $this->skipVal = null;
+        $this->with = [];
+        $this->relationFields = [];
+    }
+
+    /** Resolve database name from collection string if provided as db.collection; else default to 'app'. */
+    private function databaseName(): string
+    {
+        // Allow subclasses to define "db.collection" in collection() if they want to target a specific DB quickly
+        $col = $this->collection();
+        if (str_contains($col, '.')) {
+            return explode('.', $col, 2)[0];
+        }
+        return 'app';
+    }
+
+    // ===== Relations (MVP) =====
+
+    /** Eager load relations on next find* call. */
+    public function with(array $relations): static
+    {
+        $this->with = $relations;
+        return $this;
+    }
+
+    /** Lazy load a single relation for one DTO. */
+    public function load(AbstractDto $dto, string $relation): void
+    {
+        $this->with([$relation]);
+        $this->attachRelations([$dto]);
+        // do not call resetModifiers here to avoid wiping user sort/limit; with() is cleared in attachRelations
+    }
+
+    /** @param array<int,AbstractDto> $dtos */
+    public function loadMany(array $dtos, string $relation): void
+    {
+        if (!$dtos) return;
+        $this->with([$relation]);
+        $this->attachRelations($dtos);
+    }
+
+    /** @param array<int,AbstractDto> $parents */
+    protected function attachRelations(array $parents): void
+    {
+        if (!$parents) return;
+        $relations = $this->relations();
+        foreach ($this->with as $name) {
+            if (!isset($relations[$name])) continue;
+            $cfg = $relations[$name];
+            $type = (string)($cfg['type'] ?? '');
+            $daoClass = $cfg['dao'] ?? null;
+            if (!is_string($daoClass) || $type === '') continue;
+
+            /** @var class-string<\Pairity\NoSql\Mongo\AbstractMongoDao> $daoClass */
+            $related = new $daoClass($this->connection);
+            $relFields = $this->relationFields[$name] ?? null;
+            if ($relFields) { $related->fields(...$relFields); }
+
+            if ($type === 'hasMany' || $type === 'hasOne') {
+                $foreignKey = (string)($cfg['foreignKey'] ?? ''); // on child
+                $localKey = (string)($cfg['localKey'] ?? '_id');   // on parent
+                if ($foreignKey === '') continue;
+
+                $keys = [];
+                foreach ($parents as $p) {
+                    $arr = $p->toArray(false);
+                    if (isset($arr[$localKey])) { $keys[] = (string)$arr[$localKey]; }
+                }
+                if (!$keys) continue;
+
+                $children = $related->findAllWhereIn($foreignKey, $keys);
+                $grouped = [];
+                foreach ($children as $child) {
+                    $fk = $child->toArray(false)[$foreignKey] ?? null;
+                    if ($fk !== null) { $grouped[(string)$fk][] = $child; }
+                }
+                foreach ($parents as $p) {
+                    $arr = $p->toArray(false);
+                    $key = isset($arr[$localKey]) ? (string)$arr[$localKey] : null;
+                    $list = ($key !== null && isset($grouped[$key])) ? $grouped[$key] : [];
+                    if ($type === 'hasOne') {
+                        $p->setRelation($name, $list[0] ?? null);
+                    } else {
+                        $p->setRelation($name, $list);
+                    }
+                }
+            } elseif ($type === 'belongsTo') {
+                $foreignKey = (string)($cfg['foreignKey'] ?? ''); // on parent
+                $otherKey = (string)($cfg['otherKey'] ?? '_id');   // on related
+                if ($foreignKey === '') continue;
+
+                $ownerIds = [];
+                foreach ($parents as $p) {
+                    $arr = $p->toArray(false);
+                    if (isset($arr[$foreignKey])) { $ownerIds[] = (string)$arr[$foreignKey]; }
+                }
+                if (!$ownerIds) continue;
+
+                $owners = $related->findAllWhereIn($otherKey, $ownerIds);
+                $byId = [];
+                foreach ($owners as $o) {
+                    $id = $o->toArray(false)[$otherKey] ?? null;
+                    if ($id !== null) { $byId[(string)$id] = $o; }
+                }
+                foreach ($parents as $p) {
+                    $arr = $p->toArray(false);
+                    $fk = isset($arr[$foreignKey]) ? (string)$arr[$foreignKey] : null;
+                    $p->setRelation($name, ($fk !== null && isset($byId[$fk])) ? $byId[$fk] : null);
+                }
+            }
+        }
+        // reset eager-load request
+        $this->with = [];
+        // keep relationFields for potential subsequent relation loads within same high-level call
+    }
+}
diff --git a/src/NoSql/Mongo/Filter.php b/src/NoSql/Mongo/Filter.php
new file mode 100644
index 0000000..1ca6780
--- /dev/null
+++ b/src/NoSql/Mongo/Filter.php
@@ -0,0 +1,90 @@
+<?php
+
+namespace Pairity\NoSql\Mongo;
+
+/**
+ * Minimal fluent builder for MongoDB filters.
+ */
+final class Filter
+{
+    /** @var array<string,mixed> */
+    private array $query = [];
+
+    private function __construct(array $initial = [])
+    {
+        $this->query = $initial;
+    }
+
+    public static function make(): self
+    {
+        return new self();
+    }
+
+    /** @return array<string,mixed> */
+    public function toArray(): array
+    {
+        return $this->query;
+    }
+
+    public function whereEq(string $field, mixed $value): self
+    {
+        $this->query[$field] = $value;
+        return $this;
+    }
+
+    /** @param array<int,mixed> $values */
+    public function whereIn(string $field, array $values): self
+    {
+        $this->query[$field] = ['$in' => array_values($values)];
+        return $this;
+    }
+
+    public function gt(string $field, mixed $value): self
+    {
+        $this->op($field, '$gt', $value);
+        return $this;
+    }
+
+    public function gte(string $field, mixed $value): self
+    {
+        $this->op($field, '$gte', $value);
+        return $this;
+    }
+
+    public function lt(string $field, mixed $value): self
+    {
+        $this->op($field, '$lt', $value);
+        return $this;
+    }
+
+    public function lte(string $field, mixed $value): self
+    {
+        $this->op($field, '$lte', $value);
+        return $this;
+    }
+
+    /** Add an $or clause with an array of filters (arrays or Filter instances). */
+    public function orWhere(array $conditions): self
+    {
+        $ors = [];
+        foreach ($conditions as $c) {
+            if ($c instanceof self) {
+                $ors[] = $c->toArray();
+            } elseif (is_array($c)) {
+                $ors[] = $c;
+            }
+        }
+        if (!empty($ors)) {
+            $this->query['$or'] = $ors;
+        }
+        return $this;
+    }
+
+    private function op(string $field, string $op, mixed $value): void
+    {
+        $cur = $this->query[$field] ?? [];
+        if (!is_array($cur)) { $cur = []; }
+        $cur[$op] = $value;
+        $this->query[$field] = $cur;
+    }
+}
diff --git a/src/NoSql/Mongo/IndexManager.php b/src/NoSql/Mongo/IndexManager.php
new file mode 100644
index 0000000..2af44b7
--- /dev/null
+++ b/src/NoSql/Mongo/IndexManager.php
@@ -0,0 +1,68 @@
+<?php
+
+namespace Pairity\NoSql\Mongo;
+
+use MongoDB\Client;
+
+/**
+ * Simple Index manager for MongoDB collections.
+ */
+final class IndexManager
+{
+    private MongoConnectionInterface $connection;
+    private string $database;
+    private string $collection;
+
+    public function __construct(MongoConnectionInterface $connection, string $database, string $collection)
+    {
+        $this->connection = $connection;
+        $this->database = $database;
+        $this->collection = $collection;
+    }
+
+    /**
+     * Ensure index on keys (e.g., ['email' => 1]) with options (e.g., ['unique' => true]).
+     * Returns index name.
+     * @param array<string,int> $keys
+     * @param array<string,mixed> $options
+     */
+    public function ensureIndex(array $keys, array $options = []): string
+    {
+        $client = $this->getClient();
+        $mgr = $client->selectCollection($this->database, $this->collection)->createIndex($keys, $options);
+        return (string)$mgr;
+    }
+
+    /** Drop an index by name. */
+    public function dropIndex(string $name): void
+    {
+        $client = $this->getClient();
+        $client->selectCollection($this->database, $this->collection)->dropIndex($name);
+    }
+
+    /** @return array<int,array<string,mixed>> */
+    public function listIndexes(): array
+    {
+        $client = $this->getClient();
+        $it = $client->selectCollection($this->database, $this->collection)->listIndexes();
+        $out = [];
+        foreach ($it as $ix) {
+            $out[] = json_decode(json_encode($ix), true) ?? [];
+        }
+        return $out;
+    }
+
+    private function getClient(): Client
+    {
+        if ($this->connection instanceof MongoClientConnection) {
+            return $this->connection->getClient();
+        }
+        // Fallback: attempt to reflect getClient()
+        if (method_exists($this->connection, 'getClient')) {
+            /** @var Client $c */
+            $c = $this->connection->getClient();
+            return $c;
+        }
+        throw new \RuntimeException('IndexManager requires MongoClientConnection');
+    }
+}
diff --git a/src/NoSql/Mongo/MongoClientConnection.php b/src/NoSql/Mongo/MongoClientConnection.php
new file mode 100644
index 0000000..044522e
--- /dev/null
+++ b/src/NoSql/Mongo/MongoClientConnection.php
@@ -0,0 +1,215 @@
+<?php
+
+namespace Pairity\NoSql\Mongo;
+
+use MongoDB\Client;
+use MongoDB\BSON\ObjectId;
+use MongoDB\Driver\Session; 
+
+/**
+ * Production MongoDB adapter wrapping mongodb/mongodb Client.
+ * Implements the existing MongoConnectionInterface methods.
+ */
+class MongoClientConnection implements MongoConnectionInterface
+{
+    private Client $client;
+
+    public function __construct(Client $client)
+    {
+        $this->client = $client;
+    }
+
+    public function getClient(): Client
+    {
+        return $this->client;
+    }
+
+    public function find(string $database, string $collection, array $filter = [], array $options = []): iterable
+    {
+        $coll = $this->client->selectCollection($database, $collection);
+        $cursor = $coll->find($this->normalizeFilter($filter), $options);
+        $out = [];
+        foreach ($cursor as $doc) {
+            $out[] = $this->docToArray($doc);
+        }
+        return $out;
+    }
+
+    public function insertOne(string $database, string $collection, array $document): string
+    {
+        $coll = $this->client->selectCollection($database, $collection);
+        $result = $coll->insertOne($this->normalizeDocument($document));
+        $id = $result->getInsertedId();
+        return (string)$id;
+    }
+
+    public function updateOne(string $database, string $collection, array $filter, array $update, array $options = []): int
+    {
+        $coll = $this->client->selectCollection($database, $collection);
+        $res = $coll->updateOne($this->normalizeFilter($filter), $update, $options);
+        return $res->getModifiedCount();
+    }
+
+    public function deleteOne(string $database, string $collection, array $filter, array $options = []): int
+    {
+        $coll = $this->client->selectCollection($database, $collection);
+        $res = $coll->deleteOne($this->normalizeFilter($filter), $options);
+        return $res->getDeletedCount();
+    }
+
+    public function aggregate(string $database, string $collection, array $pipeline, array $options = []): iterable
+    {
+        $coll = $this->client->selectCollection($database, $collection);
+        $cursor = $coll->aggregate($pipeline, $options);
+        $out = [];
+        foreach ($cursor as $doc) {
+            $out[] = $this->docToArray($doc);
+        }
+        return $out;
+    }
+
+    public function upsertOne(string $database, string $collection, array $filter, array $update): string
+    {
+        $coll = $this->client->selectCollection($database, $collection);
+        // Normalize _id in filter (supports $in handled by normalizeFilter)
+        $filter = $this->normalizeFilter($filter);
+        $res = $coll->updateOne($filter, $update, ['upsert' => true]);
+        $up = $res->getUpsertedId();
+        if ($up !== null) {
+            return (string)$up;
+        }
+        // Not an upsert (matched existing). Best-effort: fetch one doc and return its _id as string.
+        $doc = $coll->findOne($filter);
+        if ($doc) {
+            $arr = $this->docToArray($doc);
+            return isset($arr['_id']) ? (string)$arr['_id'] : '';
+        }
+        return '';
+    }
+
+    public function withSession(callable $callback): mixed
+    {
+        /** @var Session $session */
+        $session = $this->client->startSession();
+        try {
+            return $callback($this, $session);
+        } finally {
+            try { $session->endSession(); } catch (\Throwable) {}
+        }
+    }
+
+    public function withTransaction(callable $callback): mixed
+    {
+        /** @var Session $session */
+        $session = $this->client->startSession();
+        try {
+            $result = $session->startTransaction();
+            $ret = $callback($this, $session);
+            $session->commitTransaction();
+            return $ret;
+        } catch (\Throwable $e) {
+            try { $session->abortTransaction(); } catch (\Throwable) {}
+            throw $e;
+        } finally {
+            try { $session->endSession(); } catch (\Throwable) {}
+        }
+    }
+
+    /** @param array<string,mixed> $filter */
+    private function normalizeFilter(array $filter): array
+    {
+        // Recursively walk the filter and convert any _id string(s) that look like 24-hex to ObjectId
+        $walker = function (&$node, $key = null) use (&$walker) {
+            if (is_array($node)) {
+                foreach ($node as $k => &$v) {
+                    $walker($v, $k);
+                }
+                return;
+            }
+            if ($key === '_id' && is_string($node) && preg_match('/^[a-f\d]{24}$/i', $node)) {
+                try { $node = new ObjectId($node); } catch (\Throwable) {}
+            }
+        };
+
+        $convertIdContainer = function (&$value) use (&$convertIdContainer) {
+            // Handle structures like ['_id' => ['$in' => ['...','...']]]
+            if (is_string($value) && preg_match('/^[a-f\d]{24}$/i', $value)) {
+                try { $value = new ObjectId($value); } catch (\Throwable) {}
+                return;
+            }
+            if (is_array($value)) {
+                foreach ($value as $k => &$v) {
+                    $convertIdContainer($v);
+                }
+            }
+        };
+
+        // Top-level traversal
+        foreach ($filter as $k => &$v) {
+            if ($k === '_id') {
+                $convertIdContainer($v);
+            } elseif (is_array($v)) {
+                // Recurse into nested boolean operators ($and/$or) etc.
+                foreach ($v as $kk => &$vv) {
+                    if ($kk === '_id') {
+                        $convertIdContainer($vv);
+                    } elseif (is_array($vv)) {
+                        foreach ($vv as $kkk => &$vvv) {
+                            if ($kkk === '_id') {
+                                $convertIdContainer($vvv);
+                            }
+                        }
+                    }
+                }
+            }
+        }
+        unset($v);
+
+        return $filter;
+    }
+
+    /** @param array<string,mixed> $doc */
+    private function normalizeDocument(array $doc): array
+    {
+        if (isset($doc['_id']) && is_string($doc['_id']) && preg_match('/^[a-f\d]{24}$/i', $doc['_id'])) {
+            try { $doc['_id'] = new ObjectId($doc['_id']); } catch (\Throwable) {}
+        }
+        return $doc;
+    }
+
+    /**
+     * Convert BSON document or array to a plain associative array, including ObjectId cast to string.
+     */
+    private function docToArray(mixed $doc): array
+    {
+        if ($doc instanceof \MongoDB\Model\BSONDocument) {
+            $doc = $doc->getArrayCopy();
+        } elseif ($doc instanceof \ArrayObject) {
+            $doc = $doc->getArrayCopy();
+        }
+        if (!is_array($doc)) {
+            return [];
+        }
+        $out = [];
+        foreach ($doc as $k => $v) {
+            if ($v instanceof ObjectId) {
+                $out[$k] = (string)$v;
+            } elseif ($v instanceof \MongoDB\BSON\UTCDateTime) {
+                $out[$k] = $v->toDateTime()->format('c');
+            } elseif ($v instanceof \MongoDB\Model\BSONDocument || $v instanceof \ArrayObject) {
+                $out[$k] = $this->docToArray($v);
+            } elseif (is_array($v)) {
+                $out[$k] = array_map(function ($item) {
+                    if ($item instanceof ObjectId) return (string)$item;
+                    if ($item instanceof \MongoDB\Model\BSONDocument || $item instanceof \ArrayObject) {
+                        return $this->docToArray($item);
+                    }
+                    return $item;
+                }, $v);
+            } else {
+                $out[$k] = $v;
+            }
+        }
+        return $out;
+    }
+}
diff --git a/src/NoSql/Mongo/MongoConnectionInterface.php b/src/NoSql/Mongo/MongoConnectionInterface.php
index 2b84e69..13b11cc 100644
--- a/src/NoSql/Mongo/MongoConnectionInterface.php
+++ b/src/NoSql/Mongo/MongoConnectionInterface.php
@@ -18,4 +18,13 @@ interface MongoConnectionInterface
 
     /** @param array<int, array<string,mixed>> $pipeline */
     public function aggregate(string $database, string $collection, array $pipeline, array $options = []): iterable;
+
+    /** @param array<string,mixed> $filter @param array<string,mixed> $update */
+    public function upsertOne(string $database, string $collection, array $filter, array $update): string;
+
+    /** Execute a callback with a client session; callback receives the connection instance and session as args. */
+    public function withSession(callable $callback): mixed;
+
+    /** Execute a callback wrapped in a driver transaction when supported. */
+    public function withTransaction(callable $callback): mixed;
 }
diff --git a/src/NoSql/Mongo/MongoConnectionManager.php b/src/NoSql/Mongo/MongoConnectionManager.php
new file mode 100644
index 0000000..f256895
--- /dev/null
+++ b/src/NoSql/Mongo/MongoConnectionManager.php
@@ -0,0 +1,58 @@
+<?php
+
+namespace Pairity\NoSql\Mongo;
+
+use MongoDB\Client;
+
+final class MongoConnectionManager
+{
+    /**
+     * Build a MongoClientConnection from config.
+     *
+     * Supported keys:
+     * - uri: full MongoDB URI (takes precedence)
+     * - hosts: string|array host(s) (default 127.0.0.1)
+     * - port: int (default 27017)
+     * - username, password
+     * - authSource: string
+     * - replicaSet: string
+     * - tls: bool
+     * - uriOptions: array (MongoDB URI options)
+     * - driverOptions: array (MongoDB driver options)
+     *
+     * @param array<string,mixed> $config
+     */
+    public static function make(array $config): MongoClientConnection
+    {
+        $uri = (string)($config['uri'] ?? '');
+        $uriOptions = (array)($config['uriOptions'] ?? []);
+        $driverOptions = (array)($config['driverOptions'] ?? []);
+
+        if ($uri === '') {
+            $hosts = $config['hosts'] ?? ($config['host'] ?? '127.0.0.1');
+            $port = (int)($config['port'] ?? 27017);
+            $hostsStr = '';
+            if (is_array($hosts)) {
+                $parts = [];
+                foreach ($hosts as $h) { $parts[] = $h . ':' . $port; }
+                $hostsStr = implode(',', $parts);
+            } else {
+                $hostsStr = (string)$hosts . ':' . $port;
+            }
+            $user = isset($config['username']) ? (string)$config['username'] : '';
+            $pass = isset($config['password']) ? (string)$config['password'] : '';
+            $auth = ($user !== '' && $pass !== '') ? ($user . ':' . $pass . '@') : '';
+
+            $query = [];
+            if (!empty($config['authSource'])) { $query['authSource'] = (string)$config['authSource']; }
+            if (!empty($config['replicaSet'])) { $query['replicaSet'] = (string)$config['replicaSet']; }
+            if (isset($config['tls'])) { $query['tls'] = $config['tls'] ? 'true' : 'false'; }
+            $qs = $query ? ('?' . http_build_query($query)) : '';
+
+            $uri = 'mongodb://' . $auth . $hostsStr . '/' . $qs;
+        }
+
+        $client = new Client($uri, $uriOptions, $driverOptions);
+        return new MongoClientConnection($client);
+    }
+}
diff --git a/tests/MongoAdapterTest.php b/tests/MongoAdapterTest.php
new file mode 100644
index 0000000..a1f70de
--- /dev/null
+++ b/tests/MongoAdapterTest.php
@@ -0,0 +1,77 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Pairity\Tests;
+
+use PHPUnit\Framework\TestCase;
+use Pairity\NoSql\Mongo\MongoConnectionManager;
+
+final class MongoAdapterTest extends TestCase
+{
+    private function hasMongoExt(): bool
+    {
+        return extension_loaded('mongodb');
+    }
+
+    public function testCrudCycle(): void
+    {
+        if (!$this->hasMongoExt()) {
+            $this->markTestSkipped('ext-mongodb not loaded');
+        }
+
+        // Attempt connection; skip if server is unavailable
+        try {
+            $conn = MongoConnectionManager::make([
+                'host' => getenv('MONGO_HOST') ?: '127.0.0.1',
+                'port' => (int)(getenv('MONGO_PORT') ?: 27017),
+            ]);
+        } catch (\Throwable $e) {
+            $this->markTestSkipped('Mongo not available: ' . $e->getMessage());
+        }
+
+        $db = 'pairity_test';
+        $col = 'widgets';
+
+        // Clean up any leftovers
+        try {
+            foreach ($conn->find($db, $col, []) as $doc) {
+                $conn->deleteOne($db, $col, ['_id' => $doc['_id']]);
+            }
+        } catch (\Throwable $e) {
+            $this->markTestSkipped('Mongo operations unavailable: ' . $e->getMessage());
+        }
+
+        // Insert
+        $id = $conn->insertOne($db, $col, [
+            'name' => 'Widget',
+            'qty' => 5,
+            'tags' => ['a','b'],
+        ]);
+        $this->assertNotEmpty($id, 'Inserted _id should be returned');
+
+        // Find by id
+        $found = $conn->find($db, $col, ['_id' => $id]);
+        $this->assertNotEmpty($found, 'Should find inserted doc');
+        $this->assertSame('Widget', $found[0]['name'] ?? null);
+
+        // Update
+        $modified = $conn->updateOne($db, $col, ['_id' => $id], ['$set' => ['qty' => 7]]);
+        $this->assertGreaterThanOrEqual(0, $modified);
+        $after = $conn->find($db, $col, ['_id' => $id]);
+        $this->assertSame(7, $after[0]['qty'] ?? null);
+
+        // Aggregate pipeline
+        $agg = $conn->aggregate($db, $col, [
+            ['$match' => ['qty' => 7]],
+            ['$project' => ['name' => 1, 'qty' => 1]],
+        ]);
+        $this->assertNotEmpty($agg);
+
+        // Delete
+        $deleted = $conn->deleteOne($db, $col, ['_id' => $id]);
+        $this->assertGreaterThanOrEqual(1, $deleted);
+        $remaining = $conn->find($db, $col, ['_id' => $id]);
+        $this->assertCount(0, $remaining);
+    }
+}
diff --git a/tests/MongoDaoTest.php b/tests/MongoDaoTest.php
new file mode 100644
index 0000000..c8d897c
--- /dev/null
+++ b/tests/MongoDaoTest.php
@@ -0,0 +1,82 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Pairity\Tests;
+
+use PHPUnit\Framework\TestCase;
+use Pairity\NoSql\Mongo\MongoConnectionManager;
+use Pairity\NoSql\Mongo\AbstractMongoDao;
+use Pairity\Model\AbstractDto;
+
+final class MongoDaoTest extends TestCase
+{
+    private function hasMongoExt(): bool
+    {
+        return extension_loaded('mongodb');
+    }
+
+    public function testCrudViaDao(): void
+    {
+        if (!$this->hasMongoExt()) {
+            $this->markTestSkipped('ext-mongodb not loaded');
+        }
+
+        // Connect (skip if server unavailable)
+        try {
+            $conn = MongoConnectionManager::make([
+                'host' => getenv('MONGO_HOST') ?: '127.0.0.1',
+                'port' => (int)(getenv('MONGO_PORT') ?: 27017),
+            ]);
+        } catch (\Throwable $e) {
+            $this->markTestSkipped('Mongo not available: ' . $e->getMessage());
+        }
+
+        // Define DTO/DAO inline for test
+        $dtoClass = new class([]) extends AbstractDto {};
+        $dtoFqcn = get_class($dtoClass);
+
+        $dao = new class($conn, $dtoFqcn) extends AbstractMongoDao {
+            private string $dto;
+            public function __construct($c, string $dto) { parent::__construct($c); $this->dto = $dto; }
+            protected function collection(): string { return 'pairity_test.widgets'; }
+            protected function dtoClass(): string { return $this->dto; }
+        };
+
+        // Clean collection
+        foreach ($dao->findAllBy([]) as $doc) {
+            $id = (string)($doc->toArray(false)['_id'] ?? '');
+            if ($id !== '') { $dao->deleteById($id); }
+        }
+
+        // Insert
+        $created = $dao->insert(['name' => 'Widget', 'qty' => 5, 'tags' => ['a','b']]);
+        $arr = $created->toArray(false);
+        $this->assertArrayHasKey('_id', $arr);
+        $id = (string)$arr['_id'];
+        $this->assertNotEmpty($id);
+
+        // Find by id
+        $found = $dao->findById($id);
+        $this->assertNotNull($found);
+        $this->assertSame('Widget', $found->toArray(false)['name'] ?? null);
+
+        // Update
+        $updated = $dao->update($id, ['qty' => 7]);
+        $this->assertSame(7, $updated->toArray(false)['qty'] ?? null);
+
+        // Projection, sorting, limit/skip
+        $list = $dao->fields('name')->sort(['name' => 1])->limit(10)->skip(0)->findAllBy([]);
+        $this->assertNotEmpty($list);
+        $this->assertArrayHasKey('name', $list[0]->toArray(false));
+
+        // Dynamic helper findOneByName
+        $one = $dao->findOneByName('Widget');
+        $this->assertNotNull($one);
+
+        // Delete
+        $deleted = $dao->deleteById($id);
+        $this->assertGreaterThanOrEqual(1, $deleted);
+        $this->assertNull($dao->findById($id));
+    }
+}
diff --git a/tests/MongoRelationsTest.php b/tests/MongoRelationsTest.php
new file mode 100644
index 0000000..aaf8c9c
--- /dev/null
+++ b/tests/MongoRelationsTest.php
@@ -0,0 +1,90 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Pairity\Tests;
+
+use PHPUnit\Framework\TestCase;
+use Pairity\NoSql\Mongo\MongoConnectionManager;
+use Pairity\NoSql\Mongo\AbstractMongoDao;
+use Pairity\Model\AbstractDto;
+
+final class MongoRelationsTest extends TestCase
+{
+    private function hasMongoExt(): bool { return \extension_loaded('mongodb'); }
+
+    public function testEagerAndLazyRelations(): void
+    {
+        if (!$this->hasMongoExt()) { $this->markTestSkipped('ext-mongodb not loaded'); }
+
+        // Connect (skip if server unavailable)
+        try {
+            $conn = MongoConnectionManager::make([
+                'host' => \getenv('MONGO_HOST') ?: '127.0.0.1',
+                'port' => (int)(\getenv('MONGO_PORT') ?: 27017),
+            ]);
+        } catch (\Throwable $e) {
+            $this->markTestSkipped('Mongo not available: ' . $e->getMessage());
+        }
+
+        // Inline DTO classes
+        $userDto = new class([]) extends AbstractDto {};
+        $userDtoClass = \get_class($userDto);
+        $postDto = new class([]) extends AbstractDto {};
+        $postDtoClass = \get_class($postDto);
+
+        // Inline DAOs with relations
+        $UserDao = new class($conn, $userDtoClass) extends AbstractMongoDao {
+            private string $dto; public function __construct($c, string $dto) { parent::__construct($c); $this->dto = $dto; }
+            protected function collection(): string { return 'pairity_test.users_rel'; }
+            protected function dtoClass(): string { return $this->dto; }
+            protected function relations(): array { return [
+                'posts' => [ 'type' => 'hasMany', 'dao' => get_class($this->makePostDao()), 'foreignKey' => 'user_id', 'localKey' => '_id' ],
+            ]; }
+            private function makePostDao(): object { return new class($this->getConnection(), 'stdClass') extends AbstractMongoDao {
+                private string $dto; public function __construct($c, string $dto) { parent::__construct($c); $this->dto = $dto; }
+                protected function collection(): string { return 'pairity_test.posts_rel'; }
+                protected function dtoClass(): string { return $this->dto; }
+            }; }
+        };
+
+        $PostDao = new class($conn, $postDtoClass) extends AbstractMongoDao {
+            private string $dto; public function __construct($c, string $dto) { parent::__construct($c); $this->dto = $dto; }
+            protected function collection(): string { return 'pairity_test.posts_rel'; }
+            protected function dtoClass(): string { return $this->dto; }
+            protected function relations(): array { return [
+                'user' => [ 'type' => 'belongsTo', 'dao' => get_class($this->makeUserDao()), 'foreignKey' => 'user_id', 'otherKey' => '_id' ],
+            ]; }
+            private function makeUserDao(): object { return new class($this->getConnection(), 'stdClass') extends AbstractMongoDao {
+                private string $dto; public function __construct($c, string $dto) { parent::__construct($c); $this->dto = $dto; }
+                protected function collection(): string { return 'pairity_test.users_rel'; }
+                protected function dtoClass(): string { return $this->dto; }
+            }; }
+        };
+
+        // Instantiate concrete DAOs for use
+        $userDao = new $UserDao($conn, $userDtoClass);
+        $postDao = new $PostDao($conn, $postDtoClass);
+
+        // Clean
+        foreach ($postDao->findAllBy([]) as $p) { $postDao->deleteById((string)($p->toArray(false)['_id'] ?? '')); }
+        foreach ($userDao->findAllBy([]) as $u) { $userDao->deleteById((string)($u->toArray(false)['_id'] ?? '')); }
+
+        // Seed one user and two posts
+        $u = $userDao->insert(['email' => 'r@example.com', 'name' => 'Rel']);
+        $uid = (string)$u->toArray(false)['_id'];
+        $postDao->insert(['title' => 'A', 'user_id' => $uid]);
+        $postDao->insert(['title' => 'B', 'user_id' => $uid]);
+
+        // Eager load posts on users
+        $users = $userDao->with(['posts'])->findAllBy([]);
+        $this->assertNotEmpty($users);
+        $this->assertIsArray($users[0]->toArray(false)['posts'] ?? null);
+
+        // Lazy load belongsTo on a post
+        $one = $postDao->findOneBy(['title' => 'A']);
+        $this->assertNotNull($one);
+        $postDao->load($one, 'user');
+        $this->assertNotNull($one->toArray(false)['user'] ?? null);
+    }
+}