NotImplementedError：此表达式不支持运算符“getitem”

Question

I wrote an SQL query to create a view for the database I'm working on and the query works as expected:

CREATE OR REPLACE VIEW public.current_model
AS SELECT pm.model_id,
    mp.name,
    ga.pit,
    pm.description,
    pm.created_datetime,
    pm.folder_full_path,
    pm.status
   FROM prediction_model pm
     JOIN material_property mp ON pm.property_id = mp.auto_id
     JOIN geographic_applicability ga ON ga.model_id = pm.auto_id
  WHERE pm.status::text <> 'INACTIVE'::text AND (pm.created_datetime IN ( SELECT max(pm2.created_datetime) AS max
           FROM prediction_model pm2
             JOIN geographic_applicability ga2 ON ga2.model_id = pm2.auto_id
             JOIN material_property mp2 ON pm2.property_id = mp2.auto_id
          GROUP BY mp2.name, ga2.pit));

Now I need to add this view to the sqlalchemy schema for the database. I have successfully implemented couple other views, but this one gets me lost (mind you, I have almost no previous sqlalchemy experience). This is what I have come up with so far:

    @classmethod
    def current_model_view(cls):
        j1 = cls.prediction_model.join(cls.material_property,
                                       cls.prediction_model.c.property_id == cls.material_property.c.auto_id)
        j2 = j1.join(cls.geographic_applicability,
                     cls.prediction_model.c.auto_id == cls.geographic_applicability.c.model_id)

        s1 = cls.prediction_model.join(cls.geographic_applicability,
                                       cls.geographic_applicability.c.model_id == cls.prediction_model.c.auto_id)
        s2 = s1.join(cls.material_property, cls.prediction_model.c.property_id == cls.material_property.c.auto_id)

        max_date = select(max(cls.prediction_model.c.created_datetime)).\
            select_from(s2).group_by(cls.material_property.c.name, cls.geographic_applicability.c.pit)

        view_selectable = select([cls.material_property.c.name,
                                  cls.geographic_applicability.c.pit,
                                  cls.prediction_model.c.description,
                                  cls.prediction_model.c.created_datetime,
                                  cls.prediction_model.c.folder_full_path,
                                  cls.prediction_model.c.status]).\
            select_from(j2).\
            where(and_(not_(cls.prediction_model.c.status == 'INACTIVE'),
                       cls.prediction_model.c.created_datetime.in_(max_date)))
        return view_selectable

But this gives me the following error:

Traceback (most recent call last):
  File "C:/python/mhi_versionist/mhi_versionist/data_base/tracker_database_schema.py", line 320, in <module>
    create_view('current_model', ModelDB.current_model_view(), ModelDB.model_db_meta)
  File "C:/python/mhi_versionist/mhi_versionist/data_base/tracker_database_schema.py", line 154, in current_model_view
    max_date = select(max(cls.prediction_model.c.created_datetime)).\
  File "C:\python\mhi_versionist\venv2\lib\site-packages\sqlalchemy\sql\operators.py", line 432, in __getitem__
    return self.operate(getitem, index)
  File "C:\python\mhi_versionist\venv2\lib\site-packages\sqlalchemy\sql\elements.py", line 762, in operate
    return op(self.comparator, *other, **kwargs)
  File "C:\python\mhi_versionist\venv2\lib\site-packages\sqlalchemy\sql\operators.py", line 432, in __getitem__
    return self.operate(getitem, index)
  File "<string>", line 1, in <lambda>
  File "C:\python\mhi_versionist\venv2\lib\site-packages\sqlalchemy\sql\type_api.py", line 67, in operate
    return o[0](self.expr, op, *(other + o[1:]), **kwargs)
  File "C:\python\mhi_versionist\venv2\lib\site-packages\sqlalchemy\sql\default_comparator.py", line 237, in _getitem_impl
    _unsupported_impl(expr, op, other, **kw)
  File "C:\python\mhi_versionist\venv2\lib\site-packages\sqlalchemy\sql\default_comparator.py", line 241, in _unsupported_impl
    raise NotImplementedError(
NotImplementedError: Operator 'getitem' is not supported on this expression

Not sure how to solve this.

Answer 1

It looks like in the following line:

max_date = select(max(cls.prediction_model.c.created_datetime)).\

you're using Python's max function, rather than passing the sql max function. Can you try using max from sqlalchemy instead? It sits in the func module. So write:

max_date = select([func.max(cls.prediction_model.c.created_datetime)]).\

Note that you also need to use a list in your select query, hence the added [ ] .

On another note, if the ForeignKey relationships have been defined correctly in your table definitions, you don't need to tell sqlalchemy explicitely how to do the join. So in that case you could get away with something like:

    j1 = cls.prediction_model.join(cls.material_property).join(cls.geographic_applicability)
    s1 = cls.prediction_model.join(cls.geographic_applicability).join(cls.material_property)