- Fixed regression from 0.8.3 as a result of :ticket:`2818`

where :meth:`.Query.exists` wouldn't work on a query that only
had a :meth:`.Query.select_from` entry but no other entities.
re: #2818 fixes #2995

3 files changed, 28 insertions, 1 deletions

diff --git a/doc/build/changelog/changelog_08.rst b/doc/build/changelog/changelog_08.rst
index a9cd75bae..7616a3f10 100644
--- a/doc/build/changelog/changelog_08.rst
+++ b/doc/build/changelog/changelog_08.rst
@@ -12,6 +12,15 @@
     :version: 0.8.6
 
     .. change::
+        :tags: bug, orm
+        :tickets: 2995,
+        :versions: 0.9.4
+
+        Fixed regression from 0.8.3 as a result of :ticket:`2818`
+        where :meth:`.Query.exists` wouldn't work on a query that only
+        had a :meth:`.Query.select_from` entry but no other entities.
+
+    .. change::
         :tags: bug, general
         :tickets: 2986
         :versions: 0.9.4
diff --git a/lib/sqlalchemy/orm/query.py b/lib/sqlalchemy/orm/query.py
index 0f5b8bf88..afcbf3500 100644
--- a/lib/sqlalchemy/orm/query.py
+++ b/lib/sqlalchemy/orm/query.py
@@ -2543,7 +2543,14 @@ class Query(object):
         .. versionadded:: 0.8.1
 
         """
-        return sql.exists(self.with_labels().statement.with_only_columns(['1']))
+
+        # .add_columns() for the case that we are a query().select_from(X),
+        # so that ".statement" can be produced (#2995) but also without
+        # omitting the FROM clause from a query(X) (#2818);
+        # .with_only_columns() after we have a core select() so that
+        # we get just "SELECT 1" without any entities.
+        return sql.exists(self.add_columns('1').with_labels().
+                                statement.with_only_columns(['1']))
 
     def count(self):
         """Return a count of rows this Query would return.
diff --git a/test/orm/test_query.py b/test/orm/test_query.py
index 0564e5fea..312a8bc72 100644
--- a/test/orm/test_query.py
+++ b/test/orm/test_query.py
@@ -1798,6 +1798,17 @@ class ExistsTest(QueryTest, AssertsCompiledSQL):
             ') AS anon_1'
         )
 
+    def test_exists_w_select_from(self):
+        User = self.classes.User
+        sess = create_session()
+
+        q1 = sess.query().select_from(User).exists()
+        self.assert_compile(sess.query(q1),
+            'SELECT EXISTS ('
+                'SELECT 1 FROM users'
+            ') AS anon_1'
+        )
+
 
 class CountTest(QueryTest):
     def test_basic(self):