Support relation by relative_url too.

Using uid in some cases can result in joins with not supported tables, so allow
to configure category level constraint check in order to cover all cases.

product/ERP5Type/Core/CategoryRelatedMembershipArityConstraint.py

@@ -67,8 +67,12 @@ class CategoryRelatedMembershipArityConstraint(CategoryMembershipArityConstraint
     # a string
     assert len(base_category_list) != 0
 
-    sql_kw = {'portal_type': portal_type_list,
-              '%s_uid' % base_category_list[0]: obj.getUid()}
+    base_category = base_category_list[0]
+    sql_kw = {'portal_type': portal_type_list}
+    if base_category.endswith('_relative_url'):
+      sql_kw['%s' % base_category] =  obj.getRelativeUrl()
+    else:
+      sql_kw['%s_uid' % base_category_list[0]] = obj.getUid()
 
     sql_kw.update(self._getExpressionValue(obj, self.getFilterParameter()))