Add more efficient way to export all records in a very large table

[kevindice]

Say I have a table with 2.5 million rows - the way this library works is that it sends a list of primary keys of all objects to export in the job-creation POST request. For very large sets of objects, uploading megabytes of primary keys can be quite slow or even run up against POST request body-size limits or other misc edge cases.

So, I think it could be useful to efficiently handle the special case of exporting everything.

Perhaps this looks like one of the django-admin gray action buttons w/ a confirmation page which creates the job.

Changes likely required:

• possible modification to the export job model to represent "all"
• adding a confirmation view page for exporting all
• adding a django admin mixin to add the grey action button
• docs update

Happy to send a PR if you think this is a good addition.

[timthelion]

OK, seems reasonable. I'm not sure if it's a good idea to make things so final in the design that there would be a separate code path just for loading all. I wonder what the security implications would be if we were to make it so if "queryset" https://github.com/auto-mat/django-import-export-celery/blob/master/import_export_celery/models/exportjob.py#L80 is a json dict rather than a json list, it is passed as a set of **kwargs to .filter(). That way saving an empty json dict {} to queryset would export all entries. This would be better because in the future people could use this for all sorts of fancy pants shinanigans.

[timthelion]

We actually need this too. Are you going to make a try at this or do I need to do it?

[timthelion]

Can I have a vote @kevindice @PetrKudy @Goldziher on the best solution to this? Would you be happy with #43 ?