adding Apex tests

force-app/main/default/classes/TestOrderController.cls

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+@isTest
+public class TestOrderController {
+
+    public static Order__c createOrder() {
+        Account acc = new Account(
+            Name='Sample Account'
+        );
+        insert acc;
+
+        Order__c newOrder = new Order__c(
+            Account__c=acc.Id
+        );
+        insert newOrder;
+        return newOrder;
+    }
+
+    public static void createOrderItem(Id orderId) {
+        Product__c p = new Product__c(
+            Name='Sample Product'
+        );
+        insert p;
+        Order_Item__c orderItem = new Order_Item__c(
+            Order__c=orderId,
+            Product__c=p.Id
+        );
+        insert orderItem;
+    }
+
+    static testMethod void testGetOrderItems() {
+        Order__c newOrder = TestOrderController.createOrder();
+        Id orderId = newOrder.Id;
+        TestOrderController.createOrderItem(orderId);
+        Test.startTest();
+        List<Order_Item__c> orderItems = OrderController.getOrderItems(orderId);
+        Test.stopTest();
+        System.assertEquals(orderItems.size(), 1);
+    }
+
+}

force-app/main/default/classes/TestOrderController.cls-meta.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<ApexClass xmlns="urn:metadata.tooling.soap.sforce.com" fqn="TestOrderController">
+    <apiVersion>45.0</apiVersion>
+    <status>Active</status>
+</ApexClass>

force-app/main/default/classes/TestProductController.cls

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+@isTest
+public class TestProductController {
+
+    public static void createProducts() {
+        List<Product__c> products = new List<Product__c>();
+
+        products.add(new Product__c(
+                        Name='Sample Bike 1',
+                        MSRP__c=1000,
+                        Category__c='Mountain',
+                        Level__c='Beginner',
+                        Material__c='Carbon'));
+
+        products.add(new Product__c(
+                        Name='Sample Bike 2',
+                        MSRP__c=1200,
+                        Category__c='Mountain',
+                        Level__c='Beginner',
+                        Material__c='Carbon'));
+
+        insert products;
+    }
+
+    static testMethod void testGetProducts() {
+        TestProductController.createProducts();
+        String filters = '{"searchKey":"Sample", "maxPrice":2000, "categories": ["Mountain"], "levels": ["Beginner"], "materials":["Carbon"]}, 1';
+        Test.startTest();
+        PagedResult result = ProductController.getProducts(filters, 1);
+        Test.stopTest();
+        System.assertEquals(result.records.size(), 2);
+    }
+    static testMethod void testGetSimilarProducts() {
+        TestProductController.createProducts();
+        Test.startTest();
+        String filters = '{"searchKey":"Sample", "maxPrice":2000, "categories": ["Mountain"], "levels": ["Beginner"], "materials":["Carbon"]}, 1';
+        PagedResult result = ProductController.getProducts(filters, 1);
+        Product__c productToCompare = (Product__c)result.records[0];
+        Product__c[] products = ProductController.getSimilarProducts(productToCompare.Id, null);
+        System.assertEquals(products.size(), 1);
+    }
+
+}

force-app/main/default/classes/TestProductController.cls-meta.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<ApexClass xmlns="urn:metadata.tooling.soap.sforce.com" fqn="TestProductController">
+    <apiVersion>45.0</apiVersion>
+    <status>Active</status>
+</ApexClass>