Read "Exif.Sony1.LensID" for Lens information
The firmware 2.0 of Sony a6000 is bugged and "forgets" to write to correct lens information to .JPG files (raw files are not affected), the new exiv2 version uses a workaround(http://dev.exiv2.org/issues/1153) to get it anyway. exiv2(trunk) outputs the lens information now correctly as "Exif.Sony1.LensID" but darktable seems just to read "Exif.Sony2.LensID". Therefore also a new darktable build against exiv2 trunk does still not show the correct lens information.
Sample files may be obtained from the already linked issue on the exiv2 project.