From 1c91a14176dad425b2800390a5d5d368d67f735d Mon Sep 17 00:00:00 2001
From: G Allajmi <ghaith.ger@gmail.com>
Date: Fri, 5 Dec 2025 15:43:27 +0000
Subject: [PATCH] Fix adding property before mapper configuration is complete
 #12858

---
 lib/sqlalchemy/orm/mapper.py |  6 ++-
 test/orm/test_mapper.py      | 73 ++++++++++++++++++++++++++++++++++++
 2 files changed, 78 insertions(+), 1 deletion(-)

diff --git a/lib/sqlalchemy/orm/mapper.py b/lib/sqlalchemy/orm/mapper.py
index fb40c6f10a..ef58ccde3d 100644
--- a/lib/sqlalchemy/orm/mapper.py
+++ b/lib/sqlalchemy/orm/mapper.py
@@ -2100,6 +2100,10 @@ class Mapper(
                 # to be addressable in subqueries
                 col.key = col._tq_key_label = key
 
+            # In the rare case of adding a ColumnProperty before the mapper is fully configured (e.g. deferring a reflected column)
+            if not hasattr(self, "columns") or not hasattr(self, "_props"):
+                return prop
+            
             self.columns.add(col, key)
 
             for col in prop.columns:
@@ -2299,7 +2303,7 @@ class Mapper(
 
         descriptor_props = util.preloaded.orm_descriptor_props
 
-        prop = self._props.get(key)
+        prop = self._props.get(key) if hasattr(self, "_props") else None
 
         if isinstance(prop, properties.ColumnProperty):
             return self._reconcile_prop_with_incoming_columns(
diff --git a/test/orm/test_mapper.py b/test/orm/test_mapper.py
index 8bb8bb32c2..d49141ab73 100644
--- a/test/orm/test_mapper.py
+++ b/test/orm/test_mapper.py
@@ -1030,6 +1030,79 @@ class MapperTest(_fixtures.FixtureTest, AssertsCompiledSQL):
         User()
         assert hasattr(User, "addresses")
         assert "addresses" in [p.key for p in m1._polymorphic_properties]
+    
+    @testing.variation("property_type", ["Column", "ColumnProperty"])
+    def test_add_property_before_mapping_is_complete(self, property_type, connection):
+        m = MetaData()
+        users = Table(
+            "deferred_users",
+            m,
+            Column("id", Integer, primary_key=True),
+            Column("name", String),
+            Column("new_column", String),
+        )
+        User = self.classes.User
+        class UserWithDeferred(User):
+            pass
+        
+        from sqlalchemy import event
+        # Listen for the event to be able to retrieve the mapper before completing the mapping process
+        @event.listens_for(UserWithDeferred, "instrument_class", propagate=True)
+        def before_mapper_configured(mapper, class_):
+            assert mapper.configured is False
+
+            col = mapper.local_table.c.new_column
+            if property_type.ColumnProperty:
+                mapper.add_property(col.key, deferred(col, group="deferred_group"))
+            else:
+                mapper.add_property(col.key, col)
+
+        if property_type.ColumnProperty:
+            # Make a deferred group
+            self.mapper(
+                UserWithDeferred,
+                users,
+                properties={
+                    "name": deferred(users.c.name, group="deferred_group")
+                }
+            )
+        else:
+            self.mapper(UserWithDeferred, users)
+
+        configure_mappers()
+
+        assert hasattr(UserWithDeferred, "new_column")
+
+        m.create_all(connection)
+        sess = Session(connection)
+        u = UserWithDeferred(id=1, name="testuser", new_column="test_value")
+        sess.add(u)
+        sess.commit()
+        sess.close()
+
+        sess = Session(connection)
+        user = sess.query(UserWithDeferred).filter_by(id=1).first()
+        if property_type.ColumnProperty:
+            assert "id" in user.__dict__
+            assert "new_column" not in user.__dict__
+            assert "name" not in user.__dict__
+
+            eq_(user.new_column, "test_value")
+
+            assert "new_column" in user.__dict__
+            assert "name" in user.__dict__ 
+            eq_(user.name, "testuser")
+
+
+        else:
+            assert "id" in user.__dict__
+            assert "new_column" in user.__dict__
+            assert "name" in user.__dict__
+            eq_(user.new_column, "test_value")
+            eq_(user.name, "testuser")
+
+        sess.close()
+        event.remove(UserWithDeferred, "instrument_class", before_mapper_configured)
 
     def test_replace_col_prop_w_syn(self):
         users, User = self.tables.users, self.classes.User
-- 
2.47.3