This is great, but the solution I saw (i.e.
{{#property:population|current-value=309000000}}) makes the whole
Wikidata absolutely useless.
(I asked Luca back about this, and perhaps one point is that the term
"current" is too easily misunderstood. The point is not that wikidata
should have such a property, but that the value at the time of saving
a past version of a wikipedia page is preserved. Perhaps
{{#property:population|value-when-saving-page=309000000}}
would be less easily misunderstood. nothing in Wikidata would be made
useless by this, it would work exactly like now when calling the
current page, it would only work differently when calling the
cite-thiy-version-of-a-page links. And it would allow a wikipedia
community to structure their work such that Wikipedia editors can
still curate and see changes to the values.
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However, as said above, this is just an example of a solution. It is
safe, very small processing overhead, small storage overhead and
scales well to load.
A more elegant solution would clearly be to do two things:
a) when creating a Wikipedia diff on the Wikipedia page version
history, to either show directly, or link to a Wikidata property diff
(reduced to the relevant parts as outlined in an earlier mail) in
addition to the wikitext diff of the page.
Note that it is not necessary to merge all Wikidata versions into the
Wikipedia version. When comparing two arbitraty Wikipedia page
version, it is irrelvant whether 1 or multiple Wikidata changes are
included, all corresponding changes should be shown on request. The
only necessary item is a single Wikidata indicator (operating like a
special version line) on top for cases where Wikidata properties are
changed after the last Wikipedia edit.
b) expand the property function such that for all calls of specific
(citable) page versions, it retrieves the property rendering at that
point in time from wikidata.
Gregor