This field returns no data, since it is a duplicate of logfile, which is the one currently returning the path to the task log file.
Fixed in toast-master/take_14102013
Merged in origin/master and in origin/dora-toaster.
The log_file field is removed. http://git.yoctoproject.org/cgit/cgit.cgi/poky-contrib/tree/bitbake/lib/toaster/orm/models.py?h=toaster/master