- Fixed bug where the session attachment error "object is already

attached to session X" would fail to prevent the object from
also being attached to the new session, in the case that execution
continued after the error raise occurred.
fixes #3301

3 files changed, 36 insertions, 8 deletions

diff --git a/doc/build/changelog/changelog_10.rst b/doc/build/changelog/changelog_10.rst
index b54a43aae..474cec093 100644
--- a/doc/build/changelog/changelog_10.rst
+++ b/doc/build/changelog/changelog_10.rst
@@ -24,6 +24,15 @@
     on compatibility concerns, see :doc:`/changelog/migration_10`.
 
     .. change::
+        :tags: bug, orm
+        :tickets: 3301
+
+        Fixed bug where the session attachment error "object is already
+        attached to session X" would fail to prevent the object from
+        also being attached to the new session, in the case that execution
+        continued after the error raise occurred.
+
+    .. change::
         :tags: bug, ext
         :tickets: 3219, 3240
 
diff --git a/lib/sqlalchemy/orm/session.py b/lib/sqlalchemy/orm/session.py
index f02d8b54e..bc9444040 100644
--- a/lib/sqlalchemy/orm/session.py
+++ b/lib/sqlalchemy/orm/session.py
@@ -1830,7 +1830,7 @@ class Session(_SessionClassMethods):
                 "function to send this object back to the transient state." %
                 state_str(state)
             )
-        self._before_attach(state)
+        self._before_attach(state, check_identity_map=False)
         self._deleted.pop(state, None)
         if discard_existing:
             self.identity_map.replace(state)
@@ -1910,13 +1910,12 @@ class Session(_SessionClassMethods):
         self._attach(state, include_before=True)
         state._load_pending = True
 
-    def _before_attach(self, state):
+    def _before_attach(self, state, check_identity_map=True):
         if state.session_id != self.hash_key and \
                 self.dispatch.before_attach:
             self.dispatch.before_attach(self, state.obj())
 
-    def _attach(self, state, include_before=False):
-        if state.key and \
+        if check_identity_map and state.key and \
             state.key in self.identity_map and \
                 not self.identity_map.contains_state(state):
             raise sa_exc.InvalidRequestError(
@@ -1932,10 +1931,11 @@ class Session(_SessionClassMethods):
                 "(this is '%s')" % (state_str(state),
                                     state.session_id, self.hash_key))
 
+    def _attach(self, state, include_before=False):
+
         if state.session_id != self.hash_key:
-            if include_before and \
-                    self.dispatch.before_attach:
-                self.dispatch.before_attach(self, state.obj())
+            if include_before:
+                self._before_attach(state)
             state.session_id = self.hash_key
             if state.modified and state._strong_obj is None:
                 state._strong_obj = state.obj()
diff --git a/test/orm/test_session.py b/test/orm/test_session.py
index 2aa0cd3eb..58551d763 100644
--- a/test/orm/test_session.py
+++ b/test/orm/test_session.py
@@ -505,6 +505,25 @@ class SessionStateTest(_fixtures.FixtureTest):
         assert user not in s
         assert s.query(User).count() == 0
 
+    def test_already_attached(self):
+        User = self.classes.User
+        users = self.tables.users
+        mapper(User, users)
+
+        s1 = Session()
+        s2 = Session()
+
+        u1 = User(id=1, name='u1')
+        make_transient_to_detached(u1)  # shorthand for actually persisting it
+        s1.add(u1)
+
+        assert_raises_message(
+            sa.exc.InvalidRequestError,
+            "Object '<User.*?>' is already attached to session",
+            s2.add, u1
+        )
+        assert u1 not in s2
+        assert not s2.identity_map.keys()
 
     @testing.uses_deprecated()
     def test_identity_conflict(self):
@@ -562,7 +581,7 @@ class SessionStateTest(_fixtures.FixtureTest):
         assert u2 is not None and u2 is not u1
         assert u2 in sess
 
-        assert_raises(Exception, lambda: sess.add(u1))
+        assert_raises(AssertionError, lambda: sess.add(u1))
 
         sess.expunge(u2)
         assert u2 not in sess