Add MiqPreloader.polymorphic_preload_for_child_classes

[NickLaMuro]

This branch was extracted from #17354 to bring attention to the large addition to MiqPreloader, which adds a new method:

MiqPreloader.polymorphic_preload_for_child_classes

This is a specialized preloader for classes with polymorphic relationships that allows for targeted preloading for those specific class types on said polymorphic relationships. This allows for taking
advantage of those associations by allowing scopes to be applied to the specific relationships and subsequent calls to the relationship can have those query definitions applied.

This means you can do things like this:

irb> tree = ExtManagementSystem.last.fulltree_rels_arranged(:except_type => "VmOrTemplate")
#=> nodes ExtManagementSystem, EmsCluster, EmsFolder, and Host `relationship_type` records
irb> records = Relationship.flatten_arranged_rels(tree)
irb> hosts_scope = Host.select(Host.arel_table[Arel.star], :v_total_vms)
irb> preloaders_per_class = { EmsCluster => [:hosts, hosts_scope], Host => hosts_scope }
irb> MiqPreloader.polymorphic_preload_for_child_classes(records, nil, preloaders_per_class)
#=> Host records with have the `v_total_vms` column load, and hosts accessed through EmsCluster will as well

Links

• Extracted from Prevents N+1 SQL queries in miq_request_workflow.rb #17354

[miq-bot]

Checked commit NickLaMuro@2f971d5 with ruby 2.3.3, rubocop 0.52.1, haml-lint 0.20.0, and yamllint 1.10.0
2 files checked, 4 offenses detected

lib/miq_preloader.rb

• ⚠️ - Line 77, Col 115 - Lint/UnusedBlockArgument - Unused block argument - k. If it's necessary, use _ or _k as an argument name to indicate that it won't be used.
• ⚠️ - Line 77, Col 59 - Lint/UnusedBlockArgument - Unused block argument - rs. If it's necessary, use _ or _rs as an argument name to indicate that it won't be used.
• ❗ - Line 58, Col 33 - Style/BlockDelimiters - Avoid using {...} for multi-line blocks.
• ❗ - Line 98, Col 15 - Style/MethodCallWithArgsParentheses - Use parentheses for method calls with arguments.

[NickLaMuro]

The above rubocop violations existed in the code that this was extracted from (minus the Module.new {...} multi-line block one, that one just looked better). To make it more obvious the changes I was pulling from, they have been left as is for the time being.

If desired, I could make a commit to fix the rubocop violations in the offending bits in lib/extensions/ar_virtual.rb and avoid the ones presented here, but felt slightly out of scope to do so.

[NickLaMuro]

@miq-bot add_label performance
@miq-bot assign @Fryguy

@Fryguy Assigning you as I figured you would be curious about this change over all (will be at least cc'ing you in the related PRs as well). Feel free to reassign to @kbrock or someone else if you don't want to review.

cc @kbrock @syncrou

[miq-bot]

This pull request has been automatically closed because it has not been updated for at least 6 months.

Feel free to reopen this pull request if these changes are still valid.

Thank you for all your contributions!