我有下面的文档（...意味着更多的属性和getter和setter）

@Document
public class Equipment
{
...
private String name;
private String internalManufacturer;
private String externalManufacturer;
...
}

和服务中的此方法，其中传递字段的名称（代码）并设置排序。

private void setSorting(String code)
{
    ...
    List<AggregationOperation> pagedAndSortedQuery= new ArrayList<>(baseQuery);
    SortOperation sortOperation = sort(Sort.Direction.ASC, code);
    if (EquipmentsSortingField.MANUFACTURER.getCode().equals(code))
    {
        sortOperation = sort(sortingDirection, "internalManufacturer", "externalManufacturer");
        pagedAndSortedQuery.add(sortOperation);
    }
}

我想把internalManufacturer和externalManufacturer当作一个字段来排序，因为如果我像在服务中那样做，首先我得到的结果是internalManufacturer排序，然后是externalManufacturer排序。重要的是，如果externalManufacturer为空，internalManufacturer就不为空，反之亦然。
例如，我在数据库中有这个：

Equipment (externalManufacturer: John, internalManufacturer: null)
Equipment (externalManufacturer: null, internalManufacturer: Carlos)
Equipment (externalManufacturer: Ash, internalManufacturer: null)
Equipment (externalManufacturer: null, internalManufacturer: Victor)

用我的代码我得到的设备排序如下：

Equipment (externalManufacturer: null, internalManufacturer: Carlos)
Equipment (externalManufacturer: null, internalManufacturer: Victor)
Equipment (externalManufacturer: Ash, internalManufacturer: null)
Equipment (externalManufacturer: John, internalManufacturer: null)

我喜欢把它们做成这样：

Equipment (externalManufacturer: Ash, internalManufacturer: null)
Equipment (externalManufacturer: null, internalManufacturer: Carlos)
Equipment (externalManufacturer: John, internalManufacturer: null)
Equipment (externalManufacturer: null, internalManufacturer: Victor)

总之，将两个字段都排序，就像它只是一个字段一样。
我该怎么做？先谢了。