Passing objects to Celery and not querying for fresh objects is not always a bad practice. If you have millions of rows in your database, querying for them is going to slow you way down. In essence, the same reason you shouldn't use your database as the Celery backend is the same reason you might not want to query the database for fresh objects. It depends on your use case of course. Passing straight values/strings should be strongly considered too since serializing and passing whole objects when you only need a single value is not good either.
Oh absolutely values before objects. I said "serializing" more in the sense that pickle is always used for storing the arguments into the queue (or whatever the default serializer).
It always depends on your use-case but generally you want your application to behave correctly, which means it has to have correct/fresh data...you can't sacrifice correctness because of an inability to scale your database.
Yes. I think saying "you can't sacrifice correctness because of an inability to scale your database" is perhaps conveying the wrong message though. I mean, your very first point is about database scaling issues and the advantages of using something like RabbitMQ to avoid expensive SQL queries.
If you are processing a lot of data in Celery, you really want to try to avoid performing any database queries. This might mean re-architecting the system. You might for example have insert-only tables (immutable objects) to address this type of concern.
