1_BasicUnitTests.cls

  static public Boolean createBaselineData() {
		Merchandise__c m = new Merchandise__c(Name='Rack Server',Price__c=1245.99,Quantity__c=500);
		insert m;
		
		Invoice__c i = new Invoice__c();
		insert i;
		
		Line_Item__c li = new Line_Item__c(Name='1',Quantity__c=10,Merchandise__c=m.Id,Invoice__c=i.Id);
		insert li;
		
		return true;	
	}

2_BasicUnitTests.cls

static public Boolean enterDataFromText(String dataString) {
  	List<String> data = dataString.split('\n');
		List<Merchandise__c> merch= new List<Merchandise__c>();
		for(String s : data) {
			List<String> objectData = s.split(',');
			Merchandise__c m = new Merchandise__c(Name=objectData[0].replaceAll('"',''),Quantity__c=Decimal.valueOf(objectData[1]),Price__c=Decimal.valueOf(objectData[2]));
			merch.add(m);
		}
		try {
			insert merch;
			return true;
		} catch(DMLException d) {
			return false;
		}
		
	

3_BasicUnitTests.cls

//CreatedDate cannot be written, but it can be imported
List<Merchandise__c> merchandise = Test.loadData(Merchandise__.sObjectType, 'MerchData');
update merchandise;
	

4_BasicUnitTests.cls

@isTest
global class MockWarehousePullData implements HttpCalloutMock {
    // Implement this interface method 
    
    global HTTPResponse respond(HTTPRequest req) {
        HttpResponse res = new HttpResponse();
        res.setHeader('Content-Type', 'application/json');
        res.setBody('{ "Items": [ { "Name": "Super Laptop", "Price": 2999.99, "Quantity": 4 }, { "Name": "Super Duper Laptop", "Price": 3999.99, "Quantity": 4 } ] }');
        res.setStatusCode(200);
        return res;
    }
}

5_BasicUnitTests.cls

        // Set mock callout class  
        Test.setMock(HttpCalloutMock.class, new MockWarehousePullData());
        
        // Call class that utilizes this endpoint 
        WarehouseDataPull.pullData();

6_BasicUnitTests.cls

@isTest
static public void testRequiredFields() {	

		//assertion: Merchandise requires Price and Quantity
  
  }

7_BasicUnitTests.cls

System.debug('REQUIRED FIELDS:');
  	//Detect all the required fields and debug out the result.  This will help if a required field is added in the future.
		Map<String, Schema.SObjectField> describeFields = Schema.SObjectType.Merchandise__c.fields.getMap();
		Map<String, Schema.DisplayType> fieldsTypes = new Map<String, Schema.DisplayType>();
		for(String field : describeFields.keyset()){
		    Schema.DescribeFieldResult dr = describeFields.get(field).getDescribe();
		    if(dr.isCreateable() && !dr.isNillable() && !dr.isDefaultedOnCreate()) {
				System.debug('REQUIRED FIELD:' +field);
			}
		}

8_BasicUnitTests.cls

@isTest
  static public void testRequiredFields() {	
		Merchandise__c m = new Merchandise__c(Name = 'Test');

		//assertion: Merchandise requires Price and Quantity
		try {
			insert m;
		} catch (DMLException e) {
			System.assert(e.getMessage().length() > 0);
		}

		m.Quantity__c = 0;
		m.Price__c = 9.99;

		//assertion: Merchandise requires Price and Quantity
		try {
			insert m;
			System.assert(String.valueOf(m.Id).length() > 0);
		} catch (DMLException e) {
			System.debug(e.getMessage());
		}

		System.debug('REQUIRED FIELDS:');
		//Detect all the required fields and debug out the result.  This will help if a required field is added in the future.
		Map<String, Schema.SObjectField> describeFields = Schema.SObjectType.Merchandise__c.fields.getMap();
		Map<String, Schema.DisplayType> fieldsTypes = new Map<String, Schema.DisplayType>();
		for(String field : describeFields.keyset()){
		    Schema.DescribeFieldResult dr = describeFields.get(field).getDescribe();
		    if(dr.isCreateable() && !dr.isNillable() && !dr.isDefaultedOnCreate()) {
				System.debug('REQUIRED FIELD:' +field);
			}
		}
	}