smarted BELONGS, thanks Jonathan

VERSION

@@ -1 +1 @@
-Version 2.6.0-development+timestamp.2013.08.09.04.48.26
+Version 2.6.0-development+timestamp.2013.08.09.11.04.59

gluon/dal.py

@@ -1265,8 +1265,13 @@ class BaseAdapter(ConnectionPool):
     def BELONGS(self, first, second):
        if isinstance(second, str):
            return '(%s IN (%s))' % (self.expand(first), second[:-1])
-        elif not second:
+       if not second:
            return '(1=0)'
+       if isinstance(second, (list,tuple,frozenset)):
+           second = set(second) # remove duplicates, make mutable
+       if isinstance(second, set) and None in second:
+           second = second.remove(None)
+           return self.OR(self.EQ(first, None), self.BELONGS(first, second))
        items = ','.join(self.expand(item, first.type) for item in second)
        return '(%s IN (%s))' % (self.expand(first), items)