Fixed a regression introduced in where.as_sql() refactor

At least Oracle needs parentheses in negated where conditions, even if
there is only single condition negated. Fixed this by reverting to old
logic in that part of as_sql() and adding a comment about this.

I did not investigate why the parentheses are needed. The original
offending commit was bd283aa844.

django/db/models/sql/where.py

@@ -135,10 +135,13 @@ class WhereNode(tree.Node):
         conn = ' %s ' % self.connector
         sql_string = conn.join(result)
         if sql_string:
-            if len(result) > 1:
-                sql_string = '(%s)' % sql_string
             if self.negated:
-                sql_string = 'NOT %s' % sql_string
+                # Note that some backends (Oracle at least) need the
+                # parentheses even around single experssion in the
+                # negated case.
+                sql_string = 'NOT (%s)' % sql_string
+            elif len(result) > 1:
+                sql_string = '(%s)' % sql_string
         return sql_string, result_params
 
     def make_atom(self, child, qn, connection):

tests/regressiontests/queries/tests.py

@@ -2091,7 +2091,7 @@ class WhereNodeTest(TestCase):
         w = WhereNode(children=[NothingNode(), self.DummyNode()], connector='OR')
         self.assertEquals(w.as_sql(qn, connection), ('dummy', []))
         w.negate()
-        self.assertEquals(w.as_sql(qn, connection), ('NOT dummy', []))
+        self.assertEquals(w.as_sql(qn, connection), ('NOT (dummy)', []))
 
     def test_empty_nodes(self):
         qn = connection.ops.quote_name