sat plugin: migrating satelites.json dumps custom groups

[axd1967]

• Whenever the version of the datafile is changed, the custom groups are lost.

Those groups should be preserved.

The pattern is to

1. read the file format version
2. import that file using that file version's importer (as it is likely that a recent file version reader might not be able to deal with older file versions; discarding the whole file is a simple yet brutal way to deal with this issue).
3. migrate through later versions (e.g. using a fall-through switch <version> construct) that applies successive updates/transformations of the data
4. save with the new file format version number

This is a design flaw that has never been dealt with. I suppose all modules that deal with JSON data need a utility to gracefully migrate older versions. I agree that this is no trivial task.

Workaround

Version the satellite datafiles, revert to the version that does contain the custom groups, then update the TLE via GUI. (Versioning should have been done prior to upgrading Stellarium...) I don't know whether this works for other JSON files too.

Additionally

• is there an issue between here and here ? both seem to bear different version numbers. Also, it is odd that this version number is stored in three places, such as here.

[alex-w]

The version of satellites.json file is not changes since version 0.12.0 of Satellites plugin. The field creator is not using to compare the versions of data. The version of plugin is not using to compare the versions of data.

[alex-w]

I've edited file and define custom group for 2 satellites at the moment, run Stellarium and call updating data for all satellites - custom group is not lost. I don't know what you doing to lost the data.