fix polymorphic_on check

Author: John Lyu

sqlmodel/_compat.py

@@ -294,16 +294,19 @@ def sqlmodel_table_construct(
         # Override polymorphic_on default value
         mapper = inspect(cls)
         polymorphic_on = mapper.polymorphic_on
-        polymorphic_property = mapper.get_property_by_column(polymorphic_on)
-        field_info = cls.model_fields.get(polymorphic_property.key)
-        if field_info:
-            v = values.get(polymorphic_property.key)
-            # if model is inherited or polymorphic_on is not explicitly set
-            # set the polymorphic_on by default
-            if mapper.inherits or v is None:
-                setattr(
-                    self_instance, polymorphic_property.key, mapper.polymorphic_identity
-                )
+        if polymorphic_on:
+            polymorphic_property = mapper.get_property_by_column(polymorphic_on)
+            field_info = cls.model_fields.get(polymorphic_property.key)
+            if field_info:
+                v = values.get(polymorphic_property.key)
+                # if model is inherited or polymorphic_on is not explicitly set
+                # set the polymorphic_on by default
+                if mapper.inherits or v is None:
+                    setattr(
+                        self_instance,
+                        polymorphic_property.key,
+                        mapper.polymorphic_identity,
+                    )
         return self_instance
 
     def sqlmodel_validate(