negate True/False separately from other elements

Fixed issue where double negation of a boolean column wouldn't reset
the "NOT" operator.

Fixes: #4618
Change-Id: Ica280a0d6b5b0870aa2d05c4d059a1e559e6b12a
(cherry picked from commit 18f25f50353d9736e6638266585b2cb3ef7b0ea4)

1 files changed, 4 insertions, 7 deletions

diff --git a/lib/sqlalchemy/sql/elements.py b/lib/sqlalchemy/sql/elements.py
index a4623128f..b0d0feff5 100644
--- a/lib/sqlalchemy/sql/elements.py
+++ b/lib/sqlalchemy/sql/elements.py
@@ -691,9 +691,6 @@ class ColumnElement(operators.ColumnOperators, ClauseElement):
 
     def _negate(self):
         if self.type._type_affinity is type_api.BOOLEANTYPE._type_affinity:
-            # TODO: see the note in AsBoolean that it seems to assume
-            # the element is the True_() / False_() constant, so this
-            # is too broad
             return AsBoolean(self, operators.isfalse, operators.istrue)
         else:
             return super(ColumnElement, self)._negate()
@@ -3035,10 +3032,10 @@ class AsBoolean(UnaryExpression):
         return self
 
     def _negate(self):
-        # TODO: this assumes the element is the True_() or False_()
-        # object, but this assumption isn't enforced and
-        # ColumnElement._negate() can send any number of expressions here
-        return self.element._negate()
+        if isinstance(self.element, (True_, False_)):
+            return self.element._negate()
+        else:
+            return AsBoolean(self.element, self.negate, self.operator)
 
 
 class BinaryExpression(ColumnElement):