Publish(): media files must not exist

[frankenjoe]

Closes #56

Allows it to publish a new version of a database without downloading all media files first. For media files referenced in a table it is sufficient that they exist in dependencies of the previous version. In that case it is assumed they remain unchanged.

This should significantly speed up publishing, since we

1. don't have to download all media files
2. don't have to calculate their checksum

Only for media files that exist in the build folder, the checksum is calculated to check if they were altered. When a media file is altered that shares the archive with other files, missing files are automatically downloaded to the build folder so that the archive can be created.

To download only the metadata to the build folder, the argument only_metadata was added to audb.load_to().

The usage section was updated accordingly:

[codecov]

Codecov Report

Merging #216 (258badb) into master (99ef956) will not change coverage.
The diff coverage is 100.0%.

Impacted Files	Coverage Δ
audb/core/load_to.py	100.0% <100.0%> (ø)
audb/core/publish.py	100.0% <100.0%> (ø)

[hagenw]

Cool, that idea is even better then I thought, because you are not adding an argument to audb.publish(), but to audb.load_to() which avoids downloading the data already and with only_metadata we have an argument, that users know already from audb.load(), so even better.

[frankenjoe]

One great advantage is that now we only have to have the media in the build folder that is actually added / altered in that version, which makes it easier to spot the changes. I would even argue it makes sense now to have a build folder for every version rather than a shared one as we had so far.

[hagenw]

There is one part where I'm wondering if this still works with your proposal: deleting of files. Before you downloaded all files with audb.load_to() and could then simply delete media files and remove them from the tables to remove them from the database.
Is this now handled by simply remove them from the tables?

[frankenjoe]

Is this now handled by simply remove them from the tables?

Yes, it was always handled this way. You can have as many media files as you want, as long as they are not referenced in at least one table they were never uploaded.

[hagenw]

Very cool, this will indeed a big relief for updating databases, as you nearly never change all media files.

[hagenw]

I'm already excited to use this new feature :o)