diff --git a/sqlalchemy_history/reverter.py b/sqlalchemy_history/reverter.py
index dca6dd1..d529caf 100644
--- a/sqlalchemy_history/reverter.py
+++ b/sqlalchemy_history/reverter.py
@@ -67,10 +67,11 @@ def revert_relationship(self, prop):
                     value = self.revert_child(child_obj, prop)
                     if value:
                         values.append(value)
-
-                for value in getattr(self.version_parent, prop.key, []):
+                collection = getattr(self.version_parent, prop.key, [])
+                for value in list(collection):
                     if value not in values:
                         self.session.delete(value)
+                        collection.remove(value)
             else:
                 self.revert_child(getattr(self.obj, prop.key), prop)
 
diff --git a/sqlalchemy_history/unit_of_work.py b/sqlalchemy_history/unit_of_work.py
index 9f09fbe..806e479 100644
--- a/sqlalchemy_history/unit_of_work.py
+++ b/sqlalchemy_history/unit_of_work.py
@@ -5,7 +5,7 @@
 import sqlalchemy as sa
 from sqlalchemy_utils import get_primary_keys, identity
 
-from sqlalchemy_history.operation import Operations
+from sqlalchemy_history.operation import Operation, Operations
 from sqlalchemy_history.schema import update_end_tx_column
 from sqlalchemy_history.utils import (
     end_tx_column_name,
@@ -301,9 +301,13 @@ def assign_attributes(self, parent_obj, version_obj):
         :param version_obj: Version object to assign the attribute values to
 
         """
+        state = sa.inspect(parent_obj)
         for prop in versioned_column_properties(parent_obj):
-            try:
-                value = getattr(parent_obj, prop.key)
-            except sa.orm.exc.ObjectDeletedError:
+            if version_obj.operation_type == Operation.DELETE and prop.key in state.unloaded:
                 value = None
+            else:
+                try:
+                    value = getattr(parent_obj, prop.key)
+                except sa.orm.exc.ObjectDeletedError:
+                    value = None
             setattr(version_obj, prop.key, value)
diff --git a/tests/revert/test_polymorphic_relationship.py b/tests/revert/test_polymorphic_relationship.py
new file mode 100644
index 0000000..785cf95
--- /dev/null
+++ b/tests/revert/test_polymorphic_relationship.py
@@ -0,0 +1,65 @@
+import sqlalchemy as sa
+from tests import TestCase
+
+
+class TestRevertPolymorphicRelationship(TestCase):
+    def create_models(self):
+        class Car(self.Model):
+            __tablename__ = "car"
+            __versioned__ = {}
+
+            id = sa.Column(
+                sa.Integer, sa.Sequence(f"{__tablename__}_seq", start=1), autoincrement=True, primary_key=True
+            )
+            parts = sa.orm.relationship(
+                "Part", back_populates="car", cascade="all, delete-orphan", lazy="selectin"
+            )
+
+        class Part(self.Model):
+            __tablename__ = "part"
+            __versioned__ = {}
+
+            id = sa.Column(
+                sa.Integer, sa.Sequence(f"{__tablename__}_seq", start=1), autoincrement=True, primary_key=True
+            )
+            car_id = sa.Column(sa.Integer, sa.ForeignKey(Car.id))
+            car = sa.orm.relationship(Car, back_populates="parts")
+
+            type = sa.Column(sa.String(50))
+
+            __mapper_args__ = {
+                "polymorphic_identity": "part",
+                "polymorphic_on": type,
+            }
+
+        class Tire(Part):
+            __tablename__ = "tire"
+            __versioned__ = {}
+
+            id = sa.Column(sa.Integer, sa.ForeignKey(Part.id), primary_key=True)
+            radius = sa.Column(sa.Integer)
+            width = sa.Column(sa.Integer)
+
+            __mapper_args__ = {
+                "polymorphic_identity": "tire",
+            }
+
+        self.Car = Car
+        self.Part = Part
+        self.Tire = Tire
+
+    def test_revert_polymorphic_relationship(self):
+        car = self.Car()
+        self.session.add(car)
+        self.session.commit()
+
+        tire = self.Tire(radius=15, width=200)
+        car.parts.append(tire)
+        self.session.commit()
+
+        initial_version = car.versions.all()[0]
+        reverted_car = initial_version.revert(relations=["parts"])
+
+        assert len(reverted_car.parts) == 0
+
+        self.session.flush()