Merge branch '18915-pagination-with-priority-sort-repeats-results' into 'master'

Fix pagination on sorts with lots of ties ## What does this MR do? Fixes #18915. As we only order by the sorted column, we don't have any tie-breaker. Some orderings, like priority and weight, have lots of ties, so you can see duplicate results as you page through. (Timestamp columns are less susceptible to this.) ## Are there points in the code the reviewer needs to double check? I just picked `id DESC`, this could as easily be `id ASC`. ## Why was this MR needed? Postgres and MySQL don't guarantee that pagination with `LIMIT` and `OFFSET` will work as expected if the ordering isn't unique. From the Postgres docs: > When using `LIMIT`, it is important to use an `ORDER BY` clause that > constrains the result rows into a unique order. Otherwise you will get > an unpredictable subset of the query's rows Before: [1] pry(main)> issues = 1.upto(Issue.count).map { |i| Issue.sort('priority').page(i).per(1).map(&:id) }.flatten [2] pry(main)> issues.count => 81 [3] pry(main)> issues.uniq.count => 42 After: [1] pry(main)> issues = 1.upto(Issue.count).map { |i| Issue.sort('priority').page(i).per(1).map(&:id) }.flatten [2] pry(main)> issues.count => 81 [3] pry(main)> issues.uniq.count => 81 See merge request !4878

Merge branch '18915-pagination-with-priority-sort-repeats-results' into 'master'
26b95777 · Rémy Coutable · 110b2759 · d7a5a28c · 26b95777 · 26b95777
Commit 26b95777 authored Jun 23, 2016 by Rémy Coutable
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CHANGELOG CHANGELOG +1 -0

issuable.rb app/models/concerns/issuable.rb +4 -1

issuable_spec.rb spec/models/concerns/issuable_spec.rb +14 -0

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--- a/CHANGELOG
+++ b/CHANGELOG
@@ -4,6 +4,7 @@ v 8.10.0 (unreleased)
  - Wrap code blocks on Activies and Todos page. !4783 (winniehell)
  - Add Sidekiq queue duration to transaction metrics.
  - Fix MR-auto-close text added to description. !4836
+  - Fix pagination when sorting by columns with lots of ties (like priority)
  - Implement Subresource Integrity for CSS and JavaScript assets. This prevents malicious assets from loading in the case of a CDN compromise.

 v 8.9.1

--- a/app/models/concerns/issuable.rb
+++ b/app/models/concerns/issuable.rb
@@ -112,7 +112,7 @@ module Issuable
    end

    def sort(method, excluded_labels: [])
-      case method.to_s
+      sorted = case method.to_s
               when 'milestone_due_asc' then order_milestone_due_asc
               when 'milestone_due_desc' then order_milestone_due_desc
               when 'downvotes_desc' then order_downvotes_desc
@@ -121,6 +121,9 @@ module Issuable
               else
                 order_by(method)
               end
+
+      # Break ties with the ID column for pagination
+      sorted.order(id: :desc)
    end

    def order_labels_priority(excluded_labels: [])

--- a/spec/models/concerns/issuable_spec.rb
+++ b/spec/models/concerns/issuable_spec.rb
@@ -154,6 +154,20 @@ describe Issue, "Issuable" do
        expect(issues).to match_array([issue1, issue2, issue, issue3])
      end
    end
+
+    context 'when all of the results are level on the sort key' do
+      let!(:issues) do
+        10.times { create(:issue, project: project) }
+      end
+
+      it 'has no duplicates across pages' do
+        sorted_issue_ids = 1.upto(10).map do |i|
+          project.issues.sort('milestone_due_desc').page(i).per(1).first.id
+        end
+
+        expect(sorted_issue_ids).to eq(sorted_issue_ids.uniq)
+      end
+    end
  end