[Tagging] "part:wikidata=*" tag proposal for multiple elements connected to the same wikidata id

[Janko Mihelić]

Last year there was a little discussion about unique wikidata ids in the
openstreetmap database:
https://lists.openstreetmap.org/pipermail/tagging/2018-August/038249.html

It was more or less decided there was no problem with this. Nevertheless, I
think we should consider having a hard rule of "*A Wikidata item cannot be
connected to more than one OSM item*".

Problems with not enforcing this rule:

- the problem of a partially downloaded database, where one is never sure
if a wikidata item is fully downloaded unless the whole database is
downloaded.

- we could get a flood of wikidata tags where one would, for example, tag
every building in a town with the wikidata id of the town, because that
building is a part of the town. Is that wrong tagging? Well, if the above
rule is not in place, I'm not sure.

- if a road segment has two road routes that are using it, then we should
tag it as "wikidata=Q1234;Q5678". That means, if we want to find any
wikidata id, we should be prepared to parse all wikidata tags and be
prepared for semicolons. This slows down any wikidata searches

- we can't enforce some rules like "tag leisure=stadium can only be
connected to something that is, or is derived from Q483110 (Stadium) in
Wikidata" because, if we tag all the parts of an entity, we can also tag a
water fountain in the stadium, because that is a part of it.

So I propose we enforce this rule, and we tag, for example, railways only
on the route relation.

If one wants to tag all route segments with a wikidata tag, I propose a
general usage "*part:wikidata=**" which would be used when a single
wikidata tag just isn't viable. Proposal wiki page here:

https://wiki.openstreetmap.org/wiki/Proposed_features/part:wikidata

Thanks for reading,
Janko Mihelić
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