Edit the liters uom (default in its category) and change its category to
unit. You now have two references uom in the unit category. Try to set a
third one and now it fails.
This was because the constraint is implemented with an sql query but
since the new ORM the update in database are delayed. We force a
database update with flush so that the query gant get all the
informations it needs.
closesodoo/odoo#41621
X-original-commit: 68ced7972db64b22a4ff031bba0f78c06a88372d
Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>