raw SQL query (but never read afterwards). Workaround for
lp:1277899
[FIX] Preserve dangling workflow table which is in use
[RFR] Group models per table when detecting columns to purge
to prevent problems with models sharing the same table
[ADD] Allow purging of dangling data entries
[FIX] Data purging now working
[IMP] Docstrings
[FIX] Label
[FIX] Catch attempt to unlink field from nonexisting model
[RFR] Flake8
Stefan Rijnhart