Mock relationships in Apex - 02 of 05

DataLayerHandler.cls

public class DataLayerHandler {
    // DataLayer Class: 
    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();
    }
}