DM-34589: Add a way to mark which dimensions populate others.

[TallJimbo]

In the future, this will be used to make it easier to export dimension data without knowing the details of the dimension combinations that represent many-to-main joins (DM-34838).

There's no real gain to updating the dimensions configuration before that happens, but merging these changes early is useful because it will make software versions with just this change much better able to handle future data repositories that use them in their dimensions configuration, even if the associated functionality isn't available as a result.

Checklist

• ran Jenkins local lsstsw
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

Patch coverage: 95.65% and no project coverage change.

Comparison is base (4927b47) 87.81% compared to head (2e8e241) 87.81%.

❗ Current head 2e8e241 differs from pull request most recent head 0bccc2b. Consider uploading reports for the commit 0bccc2b to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #685   +/-   ##
=======================================
  Coverage   87.81%   87.81%           
=======================================
  Files         274      274           
  Lines       36026    36049   +23     
  Branches     7535     7542    +7     
=======================================
+ Hits        31635    31657   +22     
  Misses       3224     3224           
- Partials     1167     1168    +1     

Files Changed	Coverage Δ
python/lsst/daf/butler/core/dimensions/_config.py	84.81% <ø> (ø)
...ython/lsst/daf/butler/core/dimensions/_universe.py	88.48% <87.50%> (-0.06%)	⬇️
...ython/lsst/daf/butler/core/dimensions/_database.py	93.00% <100.00%> (+0.25%)	⬆️
...ython/lsst/daf/butler/core/dimensions/_elements.py	83.73% <100.00%> (+0.83%)	⬆️
tests/test_dimensions.py	97.07% <100.00%> (+0.02%)	⬆️

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[timj]

I don't understand why this can't have any tests? Why can't we have a small test dimensions universe that has a couple of elements in where one populates the other. We have special test dimensions.yaml content in pipe_base and ctrl_mpexec. Why can't there be one call to get_elements_populated_by in one of the dimensions tests?

[TallJimbo]

I can add some tests with a custom dimensions universe later in the week.

[TallJimbo]

I think I'm changing my mind (a bit) again on how far to go on this ticket - I'm thinking now that we should update the default dimensions config to use populated_by, even if it doesn't do anything yet, because that will hopefully save repos created between now and some date in the future from needing a migration. But I still don't think we want to migrate any existing repos to use it.

So, since I think the rest of the migration changes are pretty close to merging over on DM-33942, let's hold this ticket up until that merges, then update the default dimensions config on this ticket after rebasing (and add a test based on that default config then).

[timj]

#674 (DM-33942) was merged.