Fix dropping of multilinks on schema change

Special case when a previously materialized multilink (one with an
existing table) is replaced with a rev_multilink property from another
table. In that case the existing multilink tables was not dropped.

Author: schlatterbeck

roundup/backends/rdbms_common.py

@@ -571,7 +571,7 @@ def update_class(self, spec, old_spec, force=0):
         old_has = {}
         for name, prop in old_spec[1]:
             old_has[name] = 1
-            if name in spec.properties:
+            if name in spec.properties and not spec.properties[name].computed:
                 continue
 
             if prop.find('Multilink to') != -1: