Fix unique constraint error

[evelyn9191]

This commit fixes situation when the database is already filled with data and the user tries to add them again. In such a case, the script failed with IntegrityError: UNIQUE constraint failed when inserting a value

[jweckstr]

I am curious of in which situations one would like to add the same content into the database? If I recall correctly the original idea has been not to accept this behavior.

Thank you for your contributions!

[rkdarst]

I also seem to remember being designed as a one-shot thing. But your changes are exactly what I would do to allow incremental updating. I guess one use case would be "create the DB, get newer version of gtfs a week later, incrementally update" ?

I'd be careful that no other bugs appear on multiple import. For example, post-processing creating duplicate structures or multiple IDs for the same object.

I'm cautiously optimistic about this, but if you're using this successfully, then that makes me positive.

[evelyn9191]

Yes, @rkdarst, that is exactly my use case.
The GTFS data are updated once per week. My app using this library would be working offline, but when a user is online and the new data are available, they will be downloaded. I prefer upserting the data rather than deleting the current DB, because I think that in case of a failed update for any reason, the user should have access to the old (even if half-way updated) data rather than no data, as it will make it impossible for him to search a connection.

I haven't tested this yet on the post-processing as I cannot get the tests working on Windows, so, for the time being, I would leave this PR to your suggestion.

[evelyn9191]

Rebased onto master branch.

[rkdarst]

So, I think this is a good idea, assuming it works and makes no other problems. I can't think of any...

Does it work for you with various tests, no corruption from re-importing something that already is in there? If so, let's do it.

[rkdarst]

I think we should do this, it will help someone and won't hurt those doing it the old way. If problems appear later, we can fix or revert.

I just added a changelog entry (and unfortunately had to merge master in - I didn't want to rebase your branch, but feel free to if you want). Could you add a test of some sort - I don't know just what, but you can probably figure out sooner than me. Even a simple thing that tries to load the same file twice would work. Can you think of easy, more advanced tests to add?