public IEnumerable <CostBusinessAreaModel> GetCostsPerMonthPerBusinessUnit() => socket.GetAllBusinessUnits()
.Join(socket.GetAllEmployees(), b => b.Id, e => e.BusinessUnitId.Id, (b, e) => new { BusinessUnit = b, Employee = e })
.Join(socket.GetAllRelations(), be => be.Employee.Id, ve => ve.EmployeeId.Id, (be, ve) => new { BusinessUnitEmployee = be, VehicleEmployee = ve })
.Join(socket.GetAllVehicles(), beve => beve.VehicleEmployee.VehicleId.Id, v => v.Id, (beve, v) => new { beve.BusinessUnitEmployee.BusinessUnit, beve.VehicleEmployee, Vehicle = v })
.Select(m => new { m.BusinessUnit, Costs = GetCostsPerVehicle(m.VehicleEmployee, m.Vehicle) })
.SelectMany(bv => bv.Costs.Select(c => new { VehicleCost = c, bv.BusinessUnit }))
.GroupBy(cb => new { cb.VehicleCost.Month, cb.BusinessUnit })
.Select(cb => new CostBusinessAreaModel()
{
Month = CultureInfo.CurrentCulture.DateTimeFormat.GetMonthName(cb.Key.Month.Month) + " " + cb.Key.Month.Year, BusinessUnit = cb.Key.BusinessUnit, Costs = cb.Sum(c => c.VehicleCost.Costs), CostDisplay = string.Format("€ {0}", cb.Sum(c => c.VehicleCost.Costs).ToString("0.00"))
});