Failure to upgrade database from 4.* to 5.* (scheme version 7 to 8)

[digikam-ng.jg at graumannschaft.org]

Hello,

Approximately a decade worth of diligent image tagging is at risk given the 
fact that on my Debian stable/testing system, I do not manage to convert the 
mysql digikam back end in the process of upgrading to digikam 5.* - please 
help!

When starting digikam v5.* pointed at the 4.*-used database backend I get the 
following: 
> digikam.dbengine: Loading SQL code from config file
> "/usr/share/digikam/database/dbconfig.xml" digikam.dbengine: Checking XML
> version ID => expected:  3  found:  3 digikam.coredb: Core database:
> running schema update
> digikam.coredb: Core database: have a structure version  7
> digikam.coredb: Core database: makeUpdates  7  to  8
> 
> digikam.dbengine: Failure executing query:
>  ""
> 
> Error messages: "QMYSQL: Unable to execute query" "Can't DROP
> 'Albums_AlbumRoots'; check that column/key exists" 1091 2 Bound values:  ()
> digikam.dbengine: Error while executing DBAction [ "UpdateSchemaFromV7ToV8"
> ] Statement [ "ALTER TABLE Albums\n                                       
> DROP FOREIGN KEY Albums_AlbumRoots;" ] digikam.coredb: Core database:
> schema update to V 8 failed!
> digikam.coredb: Core database: cannot process schema initialization 

When explicitly trying to use the db migration tool included into 5.* I end up 
with this error: 
> Error while converting the database.
> Details: Cannot add or update a child row: a foreign key constraint fails 
...
There's an image documenting the complete error here: https://
i.stack.imgur.com/wyjNn.png (I already asked at stackexchange before seeking 
help here - no takers).

Thank you for any pointers - a not insignificant part of family history is 
locked into that database ...

Sincerely, Joh