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+.. change::
+    :tags: bug, orm
+    :tickets: 8372
+
+    Changed the attribute access method used by
+    :func:`_orm.attribute_mapped_collection` and
+    :func:`_orm.column_mapped_collection`, used when populating the dictionary,
+    to assert that the data value on the object to be used as the dictionary
+    key is actually present, and is not instead using "None" due to the
+    attribute never being actually assigned. This is used to prevent a
+    mis-population of None for a key when assigning via a backref where the
+    "key" attribute on the object is not yet assigned.
+
+    As the failure mode here is a transitory condition that is not typically
+    persisted to the database, and is easy to produce via the constructor of
+    the class based on the order in which parameters are assigned, it is very
+    possible that many applications include this behavior already which is
+    silently passed over. To accommodate for applications where this error is
+    now raised, a new parameter
+    :paramref:`_orm.attribute_mapped_collection.ignore_unpopulated_attribute`
+    is also added to both :func:`_orm.attribute_mapped_collection` and
+    :func:`_orm.column_mapped_collection` that instead causes the erroneous
+    backref assignment to be skipped.