[njakobsen]

Adds order_by_child, which sorts parents by a field of their nested children. It takes the same block as with_child:

Sunspot.search(Project) do
  order_by_child :milestones, :started_at, :desc do
    with :name, "design"
  end
end

The sort is a {!parent} block join whose score is the children's value, over {!bool} of the block's restrictions, a filter that the field has a value, and {!func} of the field. A parent with no matching child that has a value comes last in either direction. Several children combine by the smallest value ascending and the largest descending, unless :score says otherwise.

• Needs Solr 8 or later; the README says so. Solr 5.3.1 (bundled) and 6.6.6 reject the sort, and Solr 7.7.3 runs it but doesn't tie times before 1970 and gets a descending :score => :min sort wrong. Its specs are skipped before Solr 8 with skip_before_solr.
• Only single-value numeric and time fields can be sorted by; a date range field raises ArgumentError. Solr compares the scores as floats, so values very close together can tie, and it reads a negative score as zero, so times before 1970 tie with each other and negative numbers with zero. A later order_by breaks a tie.

Testing

Integration and API specs for the sort, including a parent whose matching children lack the field. Locally, in both update formats, the 418 integration examples pass on solr:9.10.1 (5 pending) and on the bundled Solr 5.3.1 (12 pending: the 10 order_by_child examples and 2 that need Solr 8). The nested documents specs also pass, with the same skips, on Solr 6.6.6 and 7.7.3.

These counts include upstream master. stolo_connect's full culturecode suite, and its referral specs on all 31 clients, also pass against this branch.

Commits

Two commits on master: the feature, with its review fixes folded in, and a spec cleanup that replaces the nested documents spec's own skip_before_solr_8 with skip_before_solr. The tree is identical to the previous tip, a94d077e.

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