> when templates (or, in the case of wikidata, properties) get deleted or renamed.
> Nobody has come up with a good solution yet.

I think we did discuss a simple, working solution: Saving the value
together with the Wikipedia page.

The major argument against that was: it is a waste of storage to
create a new Wikipedia page (perhaps daily) when property values
included in a page are changed in Wikidata. I personally value trust
and documentation of change much higher than disk storage, but even
then, there are ways to balance this. So perhaps a modified proposal
that matches the current development stage:

If an editor saves a page with {{#property:population}} the parser
looks up the current value and changes this to:
  {{#property:population|current value=2348732}}
and stores this wikitext version in the Wikipedia. The same would
apply to updating, saving {{#property:population|current
value=2348732}} may result in {{#property:population|current
value=2348700}} being saved.

This would mean no additional "waste" of storage for articles that are
regularly changed. For those that are not, one could imagine a
bot-based monthly update check to make past knowledge transparent.

I realize that this would require a pattern, where the
Wikidata-derived values would remain editable on the topic/article
pages, i.e. the property function would have to be inserted in the
template call, rather than in the template definition. Those wikidata
properties automatically called inside templates with a dynamic item
decided by the current template call would not be preserved. However,
both editing patterns would be available and it would be up to the
community of each Wikipedia to choose the preferred one.

(As I said previously: although similar to the issue of commons images
and templates, the issue at stake for Wikidata is different. Because
of the problems in preserving a transparent editing history, updates
to commons images are generally restricted to truly minor improvements
(contrast, cropping, better resolution, etc.). I am not aware of
cases, where commons images regularly are replaced with updated
content that is different in substance and thus automatically changes
all Wikipedia pages, representing different knowledge. I don't want to
exclude this, but even for changing company logos the usual solution
is to create a new name, preserving the old logo. Similarly, templates
may fail to work in old versions (big problem!), but I am not aware
that a template would render out-of-time information when viewing a
past revision. Thus, the problem of Wikidata with respect to
endangering the trust basis of Wikipedia, the version system, is
related, but different).

Gregor

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