Allow loaddata to exclude some apps or models

[Wojciech Banaś]

Would propose adding this option to the command loaddata:

  -e EXCLUDE, --exclude=EXCLUDE
                        An appname or appname.ModelName to exclude (use
                        multiple --exclude to exclude multiple apps/models).

Sometimes it can be useful if you have a file with data from multiple applications / models, and we want to import only a portion of the data.

[Anssi Kääriäinen]

I am going to mark this accepted. I don't feel strongly in one way or another about this issue, I just want to get this out of unreviewed triage stage.

[Alex Morozov]

Seems useful. Recently I've faced a similar problem. Will give it a try.

[Alex Morozov]

​https://github.com/django/django/pull/5734

[Alex Morozov]

Guys, I've got a somewhat meta question: Shall I always squash my minor fixes to PR and push --force them, or shall I leave them as separate commits? Which way is the preferred one?

[Tim Graham]

If it's a huge PR, incremental commits can make follow up review easier. In this case, the patch looks small enough that you could squash right away.

[Alex Morozov]

Thank you, @timgraham. I've pushed the updated (and squashed) PR. All lights are green. It'd be nice if you review the code. I'm not sure about the ​source argument. Looks clumsy, but I haven't thought out another way of keeping the output compatibility. Is that OK?

[Berker Peksag]

I left some review comments on GitHub.

[Tim Graham]

Left a few more comments and branch needs a rebase to merge cleanly.

[Tim Graham]

​Updated PR from Berker.

[Tim Graham <timograham@…>]

In ae2a7da8:

Fixed #20468 -- Added loaddata --exclude option.

Thanks Alex Morozov for the initial patch.