An overhaul of expression handling for special symbols particularly

with conjunctions, e.g.
``None`` :func:`.expression.null` :func:`.expression.true`
:func:`.expression.false`, including consistency in rendering NULL
in conjunctions, "short-circuiting" of :func:`.and_` and :func:`.or_`
expressions which contain boolean constants, and rendering of
boolean constants and expressions as compared to "1" or "0" for backends
that don't feature ``true``/``false`` constants. [ticket:2804]

1 files changed, 21 insertions, 2 deletions

diff --git a/lib/sqlalchemy/sql/compiler.py b/lib/sqlalchemy/sql/compiler.py
index f526203ac..2bf7d3f4a 100644
--- a/lib/sqlalchemy/sql/compiler.py
+++ b/lib/sqlalchemy/sql/compiler.py
@@ -113,6 +113,7 @@ OPERATORS = {
     operators.asc_op: ' ASC',
     operators.nullsfirst_op: ' NULLS FIRST',
     operators.nullslast_op: ' NULLS LAST',
+
 }
 
 FUNCTIONS = {
@@ -608,10 +609,16 @@ class SQLCompiler(Compiled):
         return 'NULL'
 
     def visit_true(self, expr, **kw):
-        return 'true'
+        if self.dialect.supports_native_boolean:
+            return 'true'
+        else:
+            return "1"
 
     def visit_false(self, expr, **kw):
-        return 'false'
+        if self.dialect.supports_native_boolean:
+            return 'false'
+        else:
+            return "0"
 
     def visit_clauselist(self, clauselist, order_by_select=None, **kw):
         if order_by_select is not None:
@@ -783,6 +790,18 @@ class SQLCompiler(Compiled):
             raise exc.CompileError(
                             "Unary expression has no operator or modifier")
 
+    def visit_istrue_unary_operator(self, element, operator, **kw):
+        if self.dialect.supports_native_boolean:
+            return self.process(element.element, **kw)
+        else:
+            return "%s = 1" % self.process(element.element, **kw)
+
+    def visit_isfalse_unary_operator(self, element, operator, **kw):
+        if self.dialect.supports_native_boolean:
+            return "NOT %s" % self.process(element.element, **kw)
+        else:
+            return "%s = 0" % self.process(element.element, **kw)
+
     def visit_binary(self, binary, **kw):
         # don't allow "? = ?" to render
         if self.ansi_bind_rules and \