Commit d7a5a28c authored by Sean McGivern's avatar Sean McGivern

Fix pagination on sorts with lots of ties

Postgres and MySQL don't guarantee that pagination with `LIMIT` and
`OFFSET` will work 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
parent 2de9d66f
......@@ -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 8.9.0
......
......@@ -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: [])
......
......@@ -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
......
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