Mock relationships in Apex - 05 of 05

DataLayerHandler_Test.cls

@isTest
private class DataLayerHandler_Test {

    private static MockDataLayer dataLayer;
    private static DataLayerHandler testedClass;

    static {
        dataLayer = new MockDataLayer();
    }

    @isTest
    private static void proccessDocumentsTest() {
        // consider using a Mock Data Factory, but for this example, let's create a test Account: 
        Account testAccount = new Account(
                Name = 'TestAccount', 
                Id = getFakeId(Schema.Account.SObjectType),
                Business__c = getFakeId(Schema.Business__c.SObjectType)
        );

        dataLayer.documentAccounts.add(testAccount);

        Document__c testDocument = new Document__c(
            Name = 'TestDocument',
            Id = getFakeId(Schema.Document__c.SObjectType),
            Account__r = testAccount                
        );

        dataLayer.mockDocuments.add(testDocument);

        // Now, all you need to do is to create a new instance of you handler class, but now, passing your mock data layer to the constructor!

        // Since the Interface structure is the same, the mock version is accepted by the class constructor.

        Test.startTest();
            testedClass = new DataLayerHandler(dataLayer);
            testedClass.proccessDocuments();
        Test.stopTest();

        //You can even use the Mock Repository to store the result of the method proccessDocuments(), to pass to your assertions. 

    }

    //Method to generate a fake Id based on the SObject Type: 
    public static String getFakeId(Schema.SObjectType type) {
        String result = String.valueOf(fakeIdCounter++);
        return type.getDescribe().getKeyPrefix() + '0'.repeat(12 - result.length()) + result;
    }

    public class MockDataLayer implements DataLayerHandler.IDataLayer {

        List<Account> documentAccounts = new List<Account>();
        List<Document__c> mockDocuments = new List<Document__c>();

        List<Document__c> getDocumentList(){
            return mockDocuments;
        }

    }
}