diff --git a/src/models/Order.php b/src/models/Order.php
index e89a57f..00cef30 100644
--- a/src/models/Order.php
+++ b/src/models/Order.php
@@ -122,7 +122,7 @@ public function save(): bool
         $update_stock_stm = $conn->prepare($query);
 
         foreach ($this->line_items as $line_item) {
-            if (!$line_item->validate()) {
+            if (!empty($line_item->validate())) {
                 // line item contains invalid attributes
                 $conn->rollBack();
                 $conn = null;
@@ -192,9 +192,14 @@ public function save(): bool
      *
      * @param OrderProduct $orderProduct
      * @return void
+     * @throws Exception
      */
     public function addLineItem(OrderProduct $orderProduct): void
     {
+        $errors = $orderProduct->validate();
+        if (!empty($errors)) {
+            throw new Exception("Invalid line item: " . json_encode($errors));
+        }
         $this->line_items[] = $orderProduct;
     }
 
diff --git a/src/models/Store.php b/src/models/Store.php
index e230aa8..7bb82be 100644
--- a/src/models/Store.php
+++ b/src/models/Store.php
@@ -146,6 +146,30 @@ public function validate(): array
         return $errors;
     }
 
+    /**
+     * Increments stock of a product
+     * @param int $product_id ID of product whose stock will increase
+     * @param int $quantity Amount by which stock increases
+     * @return bool Success or not
+     */
+    public function addProductStock(int $product_id, int $quantity): bool
+    {
+        $conn = self::connect();
+        $query = "INSERT INTO store_product (store_id, product_id, stock_level) VALUES (:store_id, :product_id, :quantity)
+                  ON DUPLICATE KEY UPDATE stock_level = stock_level + :quantity";
+        $params = ['store_id' => $this->store_id, 'product_id' => $product_id, 'quantity' => $quantity];
+        $stm = $conn->prepare($query);
+        $stm->execute($params);
+
+        $rows_affected = $stm->rowCount();
+        $conn = null;
+        return $rows_affected === 1;
+    }
+
+    /**
+     * @param int $product_id
+     * @return int Stock level of product. Defaults to 0.
+     */
     public function getProductStock(int $product_id): int
     {
         $query = "SELECT stock_level FROM store_product WHERE store_id = :store_id AND product_id = :product_id;";
@@ -159,6 +183,9 @@ public function getProductStock(int $product_id): int
         }
     }
 
+    /**
+     * @return Product[] Array of products which store sells.
+     */
     public function getProducts(): array
     {
         $query = "SELECT p.* FROM product p JOIN store_product sp ON p.product_id = sp.product_id WHERE sp.store_id = :store_id;";
diff --git a/tests/OrderProductTest.php b/tests/OrderProductTest.php
new file mode 100644
index 0000000..2850896
--- /dev/null
+++ b/tests/OrderProductTest.php
@@ -0,0 +1,160 @@
+<?php
+
+declare(strict_types=1);
+
+use PHPUnit\Framework\TestCase;
+use Steamy\Model\Order;
+use Steamy\Model\OrderProduct;
+use Steamy\Model\Store;
+use Steamy\Model\Client;
+use Steamy\Model\Product;
+use Steamy\Core\Database;
+use Steamy\Model\Location;
+
+class OrderProductTest extends TestCase
+{
+    use Database;
+
+    private ?Order $dummy_order;
+    private ?Client $client;
+    private ?Store $dummy_store;
+    private ?Product $dummy_product;
+    private array $line_items = [];
+
+    /**
+     * @throws Exception
+     */
+    public function setUp(): void
+    {
+        parent::setUp();
+
+        // Initialize a dummy store object for testing
+        $this->dummy_store = new Store(
+            phone_no: "987654321", // Phone number
+            address: new Location(
+                street: "Augus",
+                city: "Flacq",
+                district_id: 2,
+                latitude: 60,
+                longitude: 60
+            )
+        );
+
+        $success = $this->dummy_store->save();
+        if (!$success) {
+            $errors = $this->dummy_store->validate();
+            $error_msg = "Unable to save store to database. ";
+            if (!empty($errors)) {
+                $error_msg .= "Errors: " . implode(',', $errors);
+            } else {
+                $error_msg .= "Attributes seem to be ok as per validate().";
+            }
+
+            throw new Exception($error_msg);
+        }
+
+        // Create a dummy client
+        $this->client = new Client(
+            "john@example.com",
+            "John",
+            "Doe",
+            "john_doe",
+            "password",
+            new Location("Royal", "Curepipe", 1, 50, 50)
+        );
+        $success = $this->client->save();
+        if (!$success) {
+            throw new Exception('Unable to save client');
+        }
+
+        // Create a dummy product
+        $this->dummy_product = new Product(
+            "Latte",
+            50,
+            "latte.jpeg",
+            "A delicious latte",
+            "Beverage",
+            5.0,
+            "A cup of latte",
+            new DateTime()
+        );
+        $success = $this->dummy_product->save();
+        if (!$success) {
+            throw new Exception('Unable to save product');
+        }
+
+        // Update stock level for the product
+        $this->dummy_store->addProductStock($this->dummy_product->getProductID(), 10);
+
+        // Create dummy order line items
+        $this->line_items = [
+            new OrderProduct($this->dummy_product->getProductID(), "medium", "oat", 2, 5.0)
+        ];
+
+        // Create a dummy order
+        $this->dummy_order = new Order(
+            $this->dummy_store->getStoreID(),
+            $this->client->getUserID()
+        );
+
+        // Add line items to the order
+        foreach ($this->line_items as $line_item) {
+            $this->dummy_order->addLineItem($line_item);
+        }
+
+        $success = $this->dummy_order->save();
+        if (!$success) {
+            throw new Exception('Unable to save order');
+        }
+    }
+
+    public function tearDown(): void
+    {
+        $this->dummy_order = null;
+        $this->client = null;
+        $this->dummy_store = null;
+        $this->dummy_product = null;
+        $this->line_items = [];
+
+        // Clear all data from relevant tables
+        self::query(
+            'DELETE FROM order_product; DELETE FROM `order`; DELETE FROM client; DELETE FROM user; DELETE FROM store_product; DELETE FROM product; DELETE FROM store;'
+        );
+    }
+
+    public function testValidate(): void
+    {
+        $invalidOrderProduct = new OrderProduct(
+            product_id: $this->dummy_product->getProductID(),
+            cup_size: "extra large",  // Invalid cup size
+            milk_type: "invalid milk", // Invalid milk type
+            quantity: -1,             // Invalid quantity
+            unit_price: -2.99,        // Invalid unit price
+            order_id: $this->dummy_order->getOrderID()
+        );
+
+        $errors = $invalidOrderProduct->validate();
+
+        $this->assertArrayHasKey('quantity', $errors);
+        $this->assertArrayHasKey('cup_size', $errors);
+        $this->assertArrayHasKey('milk_type', $errors);
+        $this->assertArrayHasKey('unit_price', $errors);
+    }
+
+    public function testGetByID(): void
+    {
+        // Assuming getByID is a method that retrieves an OrderProduct by order ID and product ID
+        $retrievedOrderProduct = OrderProduct::getByID(
+            $this->dummy_order->getOrderID(),
+            $this->dummy_product->getProductID()
+        );
+
+        $this->assertNotNull($retrievedOrderProduct);
+        $this->assertEquals($this->dummy_order->getOrderID(), $retrievedOrderProduct->getOrderID());
+        $this->assertEquals($this->dummy_product->getProductID(), $retrievedOrderProduct->getProductID());
+        $this->assertEquals("medium", $retrievedOrderProduct->getCupSize());
+        $this->assertEquals("oat", $retrievedOrderProduct->getMilkType());
+        $this->assertEquals(2, $retrievedOrderProduct->getQuantity());
+        $this->assertEquals(5.0, $retrievedOrderProduct->getUnitPrice());
+    }
+}
diff --git a/tests/OrderTest.php b/tests/OrderTest.php
new file mode 100644
index 0000000..0bbda2a
--- /dev/null
+++ b/tests/OrderTest.php
@@ -0,0 +1,241 @@
+<?php
+
+declare(strict_types=1);
+
+use PHPUnit\Framework\TestCase;
+use Steamy\Model\Order;
+use Steamy\Model\OrderProduct;
+use Steamy\Model\OrderStatus;
+use Steamy\Model\Store;
+use Steamy\Model\Client;
+use Steamy\Core\Database;
+use Steamy\Model\Location;
+use Steamy\Model\Product;
+
+class OrderTest extends TestCase
+{
+    use Database;
+
+    private ?Order $dummy_order = null;
+    private ?Client $client = null;
+    private ?Store $dummy_store = null;
+    private array $line_items = [];
+
+    /**
+     * @throws Exception
+     */
+    public function setUp(): void
+    {
+        parent::setUp();
+
+        // Initialize a dummy store object for testing
+        $this->dummy_store = new Store(
+            phone_no: "987654321", // Phone number
+            address: new Location(
+                street: "Augus",
+                city: "Flacq",
+                district_id: 2,
+                latitude: 60,
+                longitude: 60
+            )
+        );
+
+        $success = $this->dummy_store->save();
+        if (!$success) {
+            $errors = $this->dummy_store->validate();
+            $error_msg = "Unable to save store to database. ";
+            if (!empty($errors)) {
+                $error_msg .= "Errors: " . implode(',', $errors);
+            } else {
+                $error_msg .= "Attributes seem to be ok as per validate().";
+            }
+
+            throw new Exception($error_msg);
+        }
+
+        // Create a dummy client
+        $this->client = new Client(
+            "john@example.com",
+            "John",
+            "Doe",
+            "john_doe",
+            "password",
+            new Location("Royal", "Curepipe", 1, 50, 50)
+        );
+        $success = $this->client->save();
+        if (!$success) {
+            throw new Exception('Unable to save client');
+        }
+
+        // Create dummy products
+        $product1 = new Product(
+            "Latte",
+            50,
+            "latte.jpeg",
+            "A delicious latte",
+            "Beverage",
+            5.0,
+            "A cup of latte",
+            new DateTime()
+        );
+        $success = $product1->save();
+        if (!$success) {
+            throw new Exception('Unable to save product 1');
+        }
+
+        $product2 = new Product(
+            "Espresso",
+            30,
+            "espresso.jpeg",
+            "A strong espresso",
+            "Beverage",
+            3.0,
+            "A cup of espresso",
+            new DateTime()
+        );
+        $success = $product2->save();
+        if (!$success) {
+            throw new Exception('Unable to save product 2');
+        }
+
+        // Add stock to the store for the products
+        $this->dummy_store->addProductStock($product1->getProductID(), 10);
+        $this->dummy_store->addProductStock($product2->getProductID(), 10);
+
+        // Create dummy order line items
+        $this->line_items = [
+            new OrderProduct($product1->getProductID(), "medium", "oat", 2, 5.0),
+            new OrderProduct($product2->getProductID(), "small", "almond", 1, 3.0)
+        ];
+
+        // Create a dummy order
+        $this->dummy_order = new Order(
+            $this->dummy_store->getStoreID(),
+            $this->client->getUserID(),
+            $this->line_items
+        );
+    }
+
+    public function tearDown(): void
+    {
+        $this->dummy_order = null;
+        $this->client = null;
+        $this->dummy_store = null;
+        $this->line_items = [];
+
+        // Clear all data from relevant tables
+        self::query(
+            'DELETE FROM order_product; DELETE FROM `order`; DELETE FROM client; DELETE FROM user; DELETE FROM store_product; DELETE FROM product; DELETE FROM store;'
+        );
+    }
+
+    public function testConstructor(): void
+    {
+        $new_order = new Order(
+            $this->dummy_store->getStoreID(),
+            $this->client->getUserID(),
+            $this->line_items
+        );
+
+        self::assertEquals($this->dummy_store->getStoreID(), $new_order->getStoreID());
+        self::assertEquals($this->client->getUserID(), $new_order->getClientID());
+        self::assertEquals(OrderStatus::PENDING, $new_order->getStatus());
+        self::assertEquals($this->line_items, $new_order->getLineItems());
+    }
+
+    public function testToArray(): void
+    {
+        $result = $this->dummy_order->toArray();
+
+        self::assertArrayHasKey('order_id', $result);
+        self::assertArrayHasKey('status', $result);
+        self::assertArrayHasKey('created_date', $result);
+        self::assertArrayHasKey('pickup_date', $result);
+        self::assertArrayHasKey('client_id', $result);
+        self::assertArrayHasKey('store_id', $result);
+
+        self::assertEquals($this->dummy_order->getOrderID(), $result['order_id']);
+        self::assertEquals($this->dummy_order->getStatus()->value, $result['status']);
+        self::assertEquals($this->dummy_order->getCreatedDate()->format('Y-m-d H:i:s'), $result['created_date']);
+        self::assertEquals($this->dummy_order->getPickupDate()?->format('Y-m-d H:i:s'), $result['pickup_date']);
+        self::assertEquals($this->dummy_order->getClientID(), $result['client_id']);
+        self::assertEquals($this->dummy_order->getStoreID(), $result['store_id']);
+    }
+
+    /**
+     * @throws Exception
+     */
+    public function testSave(): void
+    {
+        $success = $this->dummy_order->save();
+        self::assertTrue($success);
+
+        $order_id = $this->dummy_order->getOrderID();
+        self::assertGreaterThan(0, $order_id);
+
+        // Verify order in database
+        $saved_order = Order::getByID($order_id);
+        self::assertNotNull($saved_order);
+        self::assertEquals($this->dummy_order->getStoreID(), $saved_order->getStoreID());
+        self::assertEquals($this->dummy_order->getClientID(), $saved_order->getClientID());
+    }
+
+    public function testSaveWithEmptyLineItems(): void
+    {
+        $this->expectException(Exception::class);
+        $this->expectExceptionMessage('Cart cannot be empty');
+
+        $order = new Order($this->dummy_store->getStoreID(), $this->client->getUserID(), []);
+        $order->save();
+    }
+
+    /**
+     * @throws Exception
+     */
+    public function testAddLineItem(): void
+    {
+        $order = new Order($this->dummy_store->getStoreID(), $this->client->getUserID());
+        $order->addLineItem(new OrderProduct(1, "medium", "oat", 1, 5.0));
+        self::assertCount(1, $order->getLineItems());
+    }
+
+    /**
+     * @throws Exception
+     */
+    public function testGetByID(): void
+    {
+        $this->dummy_order->save();
+        $order_id = $this->dummy_order->getOrderID();
+
+        $fetched_order = Order::getByID($order_id);
+        self::assertNotNull($fetched_order);
+
+        self::assertEquals($this->dummy_order->getStoreID(), $fetched_order->getStoreID());
+        self::assertEquals($this->dummy_order->getClientID(), $fetched_order->getClientID());
+        self::assertEquals($this->dummy_order->getStatus(), $fetched_order->getStatus());
+
+        // Test getByID with invalid ID
+        self::assertNull(Order::getByID(-1));
+    }
+
+    /**
+     * @throws Exception
+     */
+    public function testCalculateTotalPrice(): void
+    {
+        $this->dummy_order->save();
+        $total_price = $this->dummy_order->calculateTotalPrice();
+
+        $expected_price = array_reduce($this->line_items, function ($carry, $item) {
+            return $carry + $item->getQuantity() * $item->getUnitPrice();
+        }, 0);
+
+        self::assertEquals($expected_price, $total_price);
+    }
+
+    public function testValidate(): void
+    {
+        $errors = $this->dummy_order->validate();
+        self::assertEmpty($errors);
+    }
+}