Remove N+1 query for Noteable association.

For each event with `Event#target_type` of "Note",
we had a query to load the associated instance `noteable`.

For example, if `Note` was related to an issue, we'd load each `Issue`
with its own query (N+1 problem).

Closes #43150.

app/models/event.rb

@@ -52,12 +52,12 @@ class Event < ActiveRecord::Base
   belongs_to :target, -> {
     # If the association for "target" defines an "author" association we want to
     # eager-load this so Banzai & friends don't end up performing N+1 queries to
-    # get the authors of notes, issues, etc.
-    if reflections['events'].active_record.reflect_on_association(:author)
-      includes(:author)
-    else
-      self
+    # get the authors of notes, issues, etc. (likewise for "noteable").
+    incs = %i(author noteable).select do |a|
+      reflections['events'].active_record.reflect_on_association(a)
     end
+
+    incs.reduce(self) { |obj, a| obj.includes(a) }
   }, polymorphic: true # rubocop:disable Cop/PolymorphicAssociations
 
   has_one :push_event_payload

changelogs/unreleased/ab-43150-users-controller-show-query-limit.yml

+---
+title: Remove N+1 query for Noteable association.
+merge_request: 17956
+author:
+type: performance