From ef327adcde54ffe19a40c7bf712b35184a696923 Mon Sep 17 00:00:00 2001
From: Mike Bayer <mike_mp@zzzcomputing.com>
Date: Wed, 14 Jun 2023 10:51:43 -0400
Subject: [PATCH] warn for other mapper property objects assigned twice

was already in place for columns via other means

A warning is emitted when an ORM :func:`_orm.relationship` and other
:class:`.MapperProperty` objects are assigned to two different class
attributes at once; only one of the attributes will be mapped.  A warning
for this condition was already in place for :class:`_schema.Column` and
:class:`_orm.mapped_column` objects.

Fixes: #3532
Change-Id: Ib8057bdf229aa92137f9b8f61b26d4008181ead6
---
 doc/build/changelog/unreleased_20/3532.rst | 10 +++
 lib/sqlalchemy/orm/mapper.py               |  9 ++
 test/orm/declarative/test_basic.py         | 96 ++++++++++++++++++++++
 3 files changed, 115 insertions(+)
 create mode 100644 doc/build/changelog/unreleased_20/3532.rst

diff --git a/doc/build/changelog/unreleased_20/3532.rst b/doc/build/changelog/unreleased_20/3532.rst
new file mode 100644
index 0000000000..9e11de9434
--- /dev/null
+++ b/doc/build/changelog/unreleased_20/3532.rst
@@ -0,0 +1,10 @@
+.. change::
+    :tags: bug, orm, declarative
+    :tickets: 3532
+
+    A warning is emitted when an ORM :func:`_orm.relationship` and other
+    :class:`.MapperProperty` objects are assigned to two different class
+    attributes at once; only one of the attributes will be mapped.  A warning
+    for this condition was already in place for :class:`_schema.Column` and
+    :class:`_orm.mapped_column` objects.
+
diff --git a/lib/sqlalchemy/orm/mapper.py b/lib/sqlalchemy/orm/mapper.py
index a54c9e9707..cf9256f63e 100644
--- a/lib/sqlalchemy/orm/mapper.py
+++ b/lib/sqlalchemy/orm/mapper.py
@@ -2154,6 +2154,15 @@ class Mapper(
                 for proxy_col in col.proxy_set:
                     self._columntoproperty[proxy_col] = prop
 
+        if getattr(prop, "key", key) != key:
+            util.warn(
+                f"ORM mapped property {self.class_.__name__}.{prop.key} being "
+                "assigned to attribute "
+                f"{key!r} is already associated with "
+                f"attribute {prop.key!r}. The attribute will be de-associated "
+                f"from {prop.key!r}."
+            )
+
         prop.key = key
 
         if setparent:
diff --git a/test/orm/declarative/test_basic.py b/test/orm/declarative/test_basic.py
index 5ba1034543..985b600f0d 100644
--- a/test/orm/declarative/test_basic.py
+++ b/test/orm/declarative/test_basic.py
@@ -10,6 +10,7 @@ from sqlalchemy import ForeignKeyConstraint
 from sqlalchemy import Index
 from sqlalchemy import inspect
 from sqlalchemy import Integer
+from sqlalchemy import literal
 from sqlalchemy import select
 from sqlalchemy import String
 from sqlalchemy import testing
@@ -39,6 +40,7 @@ from sqlalchemy.orm import Mapper
 from sqlalchemy.orm import registry
 from sqlalchemy.orm import relationship
 from sqlalchemy.orm import Session
+from sqlalchemy.orm import synonym
 from sqlalchemy.orm import synonym_for
 from sqlalchemy.orm.decl_api import add_mapped_attribute
 from sqlalchemy.orm.decl_api import DeclarativeBaseNoMeta
@@ -1441,6 +1443,100 @@ class DeclarativeMultiBaseTest(
                 y = Column("y", Integer) + 5
                 z = "im not a sqlalchemy thing"
 
+    @testing.variation(
+        "attr_type",
+        [
+            "column",
+            "mapped_column",
+            "relationship",
+            "synonym",
+            "column_property",
+        ],
+    )
+    def test_attr_assigned_to_multiple_keys(self, attr_type, decl_base):
+        """test #3532"""
+
+        column_warning = expect_warnings(
+            "On class 'A', Column object 'a' named directly multiple "
+            "times, only one will be used: a, b. Consider using "
+            "orm.synonym instead"
+        )
+
+        other_warning = expect_warnings(
+            "ORM mapped property A.a being assigned to attribute 'b' is "
+            "already associated with attribute 'a'. The attribute will be "
+            "de-associated from 'a'."
+        )
+        if attr_type.column:
+            with column_warning:
+
+                class A(decl_base):
+                    __tablename__ = "a"
+
+                    id = Column(Integer, primary_key=True)
+
+                    a = Column(Integer)
+
+                    b = a
+
+        elif attr_type.mapped_column:
+            with column_warning:
+
+                class A(decl_base):
+                    __tablename__ = "a"
+
+                    id = mapped_column(Integer, primary_key=True)
+
+                    a = mapped_column(Integer)
+
+                    b = a
+
+        elif attr_type.relationship:
+            with other_warning:
+
+                class B(decl_base):
+                    __tablename__ = "b"
+
+                    id = mapped_column(Integer, primary_key=True)
+                    aid = mapped_column(ForeignKey("a.id"))
+
+                class A(decl_base):
+                    __tablename__ = "a"
+
+                    id = mapped_column(Integer, primary_key=True)
+
+                    a = relationship("B")
+
+                    b = a
+
+                decl_base.registry.configure()
+        elif attr_type.column_property:
+            with other_warning:
+
+                class A(decl_base):
+                    __tablename__ = "a"
+
+                    id = mapped_column(Integer, primary_key=True)
+
+                    a = column_property(literal("foo") + literal("bar"))
+
+                    b = a
+
+        elif attr_type.synonym:
+            with other_warning:
+
+                class A(decl_base):
+                    __tablename__ = "a"
+
+                    id = mapped_column(Integer, primary_key=True)
+                    g = mapped_column(Integer)
+                    a = synonym("g")
+
+                    b = a
+
+        else:
+            attr_type.fail()
+
     def test_column_named_twice(self):
         with expect_warnings(
             "On class 'Foo', Column object 'x' named directly multiple "
-- 
2.39.5