Mock relationships in Apex - 03 of 05

DataLayerHandler.cls

public class DataLayerHandler {

    // First, create the dataLayer variable:
    private IDataLayer dataLayer;

    // For an empty constructor, you create a new instance of the DataLayer: 
    public DataLayerHandler() {
        this(new DataLayer());
    }

    // If the constructor contains a dataLayer as a parameter, use it in your class. 
    public DataLayerHandler(IDataLayer dataLayer) {
        this.dataLayer = dataLayer;
    }

    // Here, you can access the data using the DataLayer: 
    public void proccessDocuments() {
        List<Document__c> documentsToProcess = dataLayer.getDocumentList();        
    }

    public class DataLayer implements IDataLayer {
        List<Document__c> getDocumentList(){
            return [SELECT Id, Account__r.Business__c FROM Document__c];
        }
    }

    public interface IDataLayer {
        List<Document__c> getDocumentList();
    }
}