On Jan 24, 2011, at 6:55 AM, Massi wrote:
> Hi everyone, in my script I have to deal with a table whose number of
> columns can change at runtime. Since I have no information about the
> structure I will have to handle at a given moment, I would need to
> change dynamically the mapping during the program. If I try to re-map
> the table I obviously get the following error:
>
> sqlalchemy.exc.ArgumentError: Class ''
> already has a primary mapper defined. Use non_primary=True to create a
> non primary Mapper. clear_mappers() will remove *all* current mappers
> from all classes.
>
> On the other hand clear_mappers removes all the mappers that has been
> defined on the classes; furthermore, as the documentation says:
>
> clear_mappers` is *not* for normal use, as there is literally no valid
> usage for it outside of very specific testing scenarios
>
> Can anyone give a hint to handle correctly this situation or point me
> out if there is a better approach to face it?
First I'll note that if the use case here is a table where some human or
process is constantly adding/dropping columns and the application needs to just
keep running and auto-adjusting to the schema changes, that is completely the
wrong way to use relational databases. Its like renovating your kitchen as you
make dinner at the same time.
Secondly, assuming that use case, you want to create the classes themselves on
the fly. Use the recipe at entity name:
http://www.sqlalchemy.org/trac/wiki/UsageRecipes/EntityName
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