Add a management command which re-orders a OrderModel

[flupzor]

Very often I end up in the case where the 'order' values are no longer unique. Usually it's because of one of the fields in 'order_with_respect' to is changed, and the code does not deal well with this situation. This management command repairs the ordering, so that it can become usable again.

[imomaliev]

duplicate #174

[flupzor]

Ah, I didn't notice due to the PR title being very non-descriptive. Looks like my PR does more than the other PR and does everything which is requested there.

The only thing I see is that line 53-56 of my PR might have to be removed. This, probably, skips over the mentioned bug in the ticket.

        # Skip if none of the orderings overlap.
        order_values = list(queryset.values_list(order_field_name, flat=True))
        if len(set(order_values)) == len(order_values):
            return

I can also add a test which verifies that if gaps are introduced they are re-numbered properly. I was under the wrong assumption that django-ordered-model automatically fixes these gaps. That's why I didn't test for this.

[shuckc]

Yes this looks much better. Worth comparing the tests etc. After renunbering there should be no gaps in the numbering for each ordered group.

[flupzor]

I've included what I thought was useful from the other PR. The test didn't actually work, I modified the test to do what I think the original intent was.

I also changed the description in the README somewhat, because, I don't think it is true that a delete does not re-order in the admin (Haven't verified this). Only bulk deletes would not work (Does the admin even do bulk deletes?), or if the delete method is somehow bypassed.

I also removed the 'start_number' argument, because I don't see how that is useful.

[ronaldgrn]

When can we get this merged? Or is there an alternative solution for this problem?

[shuckc]

I've rebased this, dropped six for python 2 support, fixed up black formatting, modified the tests not to write to the command line during test runs, added assertions for management command cli output and verified tox runs clean locally. It looks like when the PR was opened @flupzor has not allowed contributors write access, so I have pushed this to a branch reorder-management-command - we might need to open a new PR to merge it.

Certainly there is demand for it - see #236 #174 #64 and since the code is disjoint from what we have there's no risk of regressions, so I would like to get it merged for the next release.

[shuckc]

Closing as taken forward on #240