- Fixed issue where a straight SELECT EXISTS query would fail to

assign the proper result type of Boolean to the result mapping, and
instead would leak column types from within the query into the
result map.  This issue exists in 0.9 and earlier as well, however
has less of an impact in those versions.  In 1.0, due to #918
this becomes a regression in that we now rely upon the result mapping
to be very accurate, else we can assign result-type processors to
the wrong column.   In all versions, this issue also has the effect
that a simple EXISTS will not apply the Boolean type handler, leading
to simple 1/0 values for backends without native boolean instead of
True/False.   The fix includes that an EXISTS columns argument
will be anon-labeled like other column expressions; a similar fix is
implemented for pure-boolean expressions like ``not_(True())``.
fixes #3372

6 files changed, 87 insertions, 16 deletions

diff --git a/doc/build/changelog/changelog_10.rst b/doc/build/changelog/changelog_10.rst
index 859adae81..52b35c931 100644
--- a/doc/build/changelog/changelog_10.rst
+++ b/doc/build/changelog/changelog_10.rst
@@ -19,6 +19,24 @@
     :version: 1.0.1
 
     .. change::
+        :tags: bug, core
+        :tickets: 3372
+
+        Fixed issue where a straight SELECT EXISTS query would fail to
+        assign the proper result type of Boolean to the result mapping, and
+        instead would leak column types from within the query into the
+        result map.  This issue exists in 0.9 and earlier as well, however
+        has less of an impact in those versions.  In 1.0, due to #918
+        this becomes a regression in that we now rely upon the result mapping
+        to be very accurate, else we can assign result-type processors to
+        the wrong column.   In all versions, this issue also has the effect
+        that a simple EXISTS will not apply the Boolean type handler, leading
+        to simple 1/0 values for backends without native boolean instead of
+        True/False.   The fix includes that an EXISTS columns argument
+        will be anon-labeled like other column expressions; a similar fix is
+        implemented for pure-boolean expressions like ``not_(True())``.
+
+    .. change::
         :tags: bug, orm
         :tickets: 3374
 
diff --git a/lib/sqlalchemy/sql/compiler.py b/lib/sqlalchemy/sql/compiler.py
index 755193552..5633159cd 100644
--- a/lib/sqlalchemy/sql/compiler.py
+++ b/lib/sqlalchemy/sql/compiler.py
@@ -1324,10 +1324,17 @@ class SQLCompiler(Compiled):
             result_expr = _CompileLabel(col_expr,
                                         elements._as_truncated(column.name),
                                         alt_names=(column.key,))
-        elif not isinstance(column,
-                            (elements.UnaryExpression, elements.TextClause)) \
-                and (not hasattr(column, 'name') or
-                     isinstance(column, functions.Function)):
+        elif (
+            not isinstance(column, elements.TextClause) and
+            (
+                not isinstance(column, elements.UnaryExpression) or
+                column.wraps_column_expression
+            ) and
+            (
+                not hasattr(column, 'name') or
+                isinstance(column, functions.Function)
+            )
+        ):
             result_expr = _CompileLabel(col_expr, column.anon_label)
         elif col_expr is not column:
             # TODO: are we sure "column" has a .name and .key here ?
@@ -1528,6 +1535,12 @@ class SQLCompiler(Compiled):
                     'need_result_map_for_compound', False)
             ) or entry.get('need_result_map_for_nested', False)
 
+        # this was first proposed as part of #3372; however, it is not
+        # reached in current tests and could possibly be an assertion
+        # instead.
+        if not populate_result_map and 'add_to_result_map' in kwargs:
+            del kwargs['add_to_result_map']
+
         if needs_nested_translation:
             if populate_result_map:
                 self._transform_result_map_for_nested_joins(
diff --git a/lib/sqlalchemy/sql/elements.py b/lib/sqlalchemy/sql/elements.py
index ca8ec1f55..6ee4053a7 100644
--- a/lib/sqlalchemy/sql/elements.py
+++ b/lib/sqlalchemy/sql/elements.py
@@ -2407,13 +2407,14 @@ class UnaryExpression(ColumnElement):
     __visit_name__ = 'unary'
 
     def __init__(self, element, operator=None, modifier=None,
-                 type_=None, negate=None):
+                 type_=None, negate=None, wraps_column_expression=False):
         self.operator = operator
         self.modifier = modifier
         self.element = element.self_group(
             against=self.operator or self.modifier)
         self.type = type_api.to_instance(type_)
         self.negate = negate
+        self.wraps_column_expression = wraps_column_expression
 
     @classmethod
     def _create_nullsfirst(cls, column):
@@ -2455,7 +2456,8 @@ class UnaryExpression(ColumnElement):
         """
         return UnaryExpression(
             _literal_as_label_reference(column),
-            modifier=operators.nullsfirst_op)
+            modifier=operators.nullsfirst_op,
+            wraps_column_expression=False)
 
     @classmethod
     def _create_nullslast(cls, column):
@@ -2496,7 +2498,8 @@ class UnaryExpression(ColumnElement):
         """
         return UnaryExpression(
             _literal_as_label_reference(column),
-            modifier=operators.nullslast_op)
+            modifier=operators.nullslast_op,
+            wraps_column_expression=False)
 
     @classmethod
     def _create_desc(cls, column):
@@ -2534,7 +2537,9 @@ class UnaryExpression(ColumnElement):
 
         """
         return UnaryExpression(
-            _literal_as_label_reference(column), modifier=operators.desc_op)
+            _literal_as_label_reference(column),
+            modifier=operators.desc_op,
+            wraps_column_expression=False)
 
     @classmethod
     def _create_asc(cls, column):
@@ -2571,7 +2576,9 @@ class UnaryExpression(ColumnElement):
 
         """
         return UnaryExpression(
-            _literal_as_label_reference(column), modifier=operators.asc_op)
+            _literal_as_label_reference(column),
+            modifier=operators.asc_op,
+            wraps_column_expression=False)
 
     @classmethod
     def _create_distinct(cls, expr):
@@ -2611,7 +2618,8 @@ class UnaryExpression(ColumnElement):
         """
         expr = _literal_as_binds(expr)
         return UnaryExpression(
-            expr, operator=operators.distinct_op, type_=expr.type)
+            expr, operator=operators.distinct_op,
+            type_=expr.type, wraps_column_expression=False)
 
     @property
     def _order_by_label_element(self):
@@ -2648,7 +2656,8 @@ class UnaryExpression(ColumnElement):
                 operator=self.negate,
                 negate=self.operator,
                 modifier=self.modifier,
-                type_=self.type)
+                type_=self.type,
+                wraps_column_expression=self.wraps_column_expression)
         else:
             return ClauseElement._negate(self)
 
@@ -2667,6 +2676,7 @@ class AsBoolean(UnaryExpression):
         self.operator = operator
         self.negate = negate
         self.modifier = None
+        self.wraps_column_expression = True
 
     def self_group(self, against=None):
         return self
diff --git a/lib/sqlalchemy/sql/selectable.py b/lib/sqlalchemy/sql/selectable.py
index f848ef6db..7d8c885ae 100644
--- a/lib/sqlalchemy/sql/selectable.py
+++ b/lib/sqlalchemy/sql/selectable.py
@@ -3343,7 +3343,8 @@ class Exists(UnaryExpression):
             s = Select(*args, **kwargs).as_scalar().self_group()
 
         UnaryExpression.__init__(self, s, operator=operators.exists,
-                                 type_=type_api.BOOLEANTYPE)
+                                 type_=type_api.BOOLEANTYPE,
+                                 wraps_column_expression=True)
 
     def select(self, whereclause=None, **params):
         return Select([self], whereclause, **params)
diff --git a/test/sql/test_compiler.py b/test/sql/test_compiler.py
index 4bdd9fcdc..75f9a7c82 100644
--- a/test/sql/test_compiler.py
+++ b/test/sql/test_compiler.py
@@ -380,7 +380,7 @@ class SelectTest(fixtures.TestBase, AssertsCompiledSQL):
         # this is native_boolean=False for default dialect
         self.assert_compile(
             select([not_(True)], use_labels=True),
-            "SELECT :param_1 = 0"
+            "SELECT :param_1 = 0 AS anon_1"
         )
 
         self.assert_compile(
@@ -561,13 +561,13 @@ class SelectTest(fixtures.TestBase, AssertsCompiledSQL):
         self.assert_compile(exists([table1.c.myid], table1.c.myid
                                    == 5).select(),
                             'SELECT EXISTS (SELECT mytable.myid FROM '
-                            'mytable WHERE mytable.myid = :myid_1)',
+                            'mytable WHERE mytable.myid = :myid_1) AS anon_1',
                             params={'mytable_myid': 5})
         self.assert_compile(select([table1, exists([1],
                                                    from_obj=table2)]),
                             'SELECT mytable.myid, mytable.name, '
                             'mytable.description, EXISTS (SELECT 1 '
-                            'FROM myothertable) FROM mytable',
+                            'FROM myothertable) AS anon_1 FROM mytable',
                             params={})
         self.assert_compile(select([table1,
                                     exists([1],
diff --git a/test/sql/test_selectable.py b/test/sql/test_selectable.py
index 3931f99e4..3390f4a77 100644
--- a/test/sql/test_selectable.py
+++ b/test/sql/test_selectable.py
@@ -2113,7 +2113,7 @@ class WithLabelsTest(fixtures.TestBase):
         self._assert_result_keys(sel, ['t1_a', 't2_b'])
 
 
-class SelectProxyTest(fixtures.TestBase):
+class ResultMapTest(fixtures.TestBase):
 
     def _fixture(self):
         m = MetaData()
@@ -2183,6 +2183,35 @@ class SelectProxyTest(fixtures.TestBase):
         assert l1 in mapping
         assert ta.c.x not in mapping
 
+    def test_column_subquery_exists(self):
+        t = self._fixture()
+        s = exists().where(t.c.x == 5).select()
+        mapping = self._mapping(s)
+        assert t.c.x not in mapping
+        eq_(
+            [type(entry[-1]) for entry in s.compile()._result_columns],
+            [Boolean]
+        )
+
+    def test_column_subquery_plain(self):
+        t = self._fixture()
+        s1 = select([t.c.x]).where(t.c.x > 5).as_scalar()
+        s2 = select([s1])
+        mapping = self._mapping(s2)
+        assert t.c.x not in mapping
+        assert s1 in mapping
+        eq_(
+            [type(entry[-1]) for entry in s2.compile()._result_columns],
+            [Integer]
+        )
+
+    def test_unary_boolean(self):
+
+        s1 = select([not_(True)], use_labels=True)
+        eq_(
+            [type(entry[-1]) for entry in s1.compile()._result_columns],
+            [Boolean]
+        )
 
 class ForUpdateTest(fixtures.TestBase, AssertsCompiledSQL):
     __dialect__ = "default"