How to test functions with complex data interactions

I am currently working on a system that performs quite a lot of reporting-style functions, which consumes many different data points and converts them into larger, sometimes flattened outputs. Most of my application is based on the repository template. Because of this, I have a set of mock repositories that I use to test scripts. The problem I am facing is that the interaction between these data points is so complex that it quickly becomes a service nightmare in order to maintain a “data layout”. Here is an example:

public class SomeReportingEntity
{
private IProductRepo ProductRepo;
private IManagerRepo ManagerRepo;
private ILocationRepo LocationRepo;
private IOrdersService OrdersService;
private IEmployeeRepo EmployeeRepo;

public ReportingEntity(IProductRepo ipr, IManagerRepo imr, ILocationRepo ilr, IOrdersService ios, 
    IEmployeeRepo ier){
        //Load these to private vars...
}

    //This is the function that I want to test...
public SomeReportingEntity GetManagerSalesByRegionReport()
{
    //Make a complex join on all sub collections.  These
    //sub collections are all under test individually.
    var MangerSalesByRegionItems = From x in ProductRepo.CurrentProducts()
                              Join y in OrdersService.FutureOrders() On ...
                              Join z in EmployeeRepo.ActiveEmployees() On  ...
                              Join a in LocationRepo.GetAllRegions() On ...
                              Join b In ManagerRepo.GetActiveManagers On ...
                              Select new SomeReportingEntity() With { ... }

    return MangerSalesByRegionItems.ToList();       
}

}

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