Add a sort order to represent "staleness"

app/models/bubble.rb

@@ -20,6 +20,7 @@ class Bubble < ApplicationRecord
   scope :indexed_by, ->(index) do
     case index
     when "most_active"    then ordered_by_activity
+    when "most_stale"     then ordered_by_staleness
     when "most_discussed" then ordered_by_comments
     when "most_boosted"   then ordered_by_boosts
     when "newest"         then reverse_chronologically

app/models/bubble/scorable.rb

@@ -5,6 +5,13 @@ module Bubble::Scorable
 
   included do
     scope :ordered_by_activity, -> { order activity_score_order: :desc }
+
+    # Staleness is measured by the amount of activity_score lost since it was last updated.
+    # The factor 0.9 is chosen to make the decay curve closer to linear; an exponential
+    # curve would make recent items appear stale very quickly, due to the sharp dropoff.
+    scope :ordered_by_staleness, -> { order Arel.sql(
+      "coalesce(activity_score * (1 - power(0.9, julianday('now') - julianday(activity_score_at))), 0) desc"
+    ) }
   end
 
   def rescore

app/models/filter/fields.rb

@@ -1,7 +1,7 @@
 module Filter::Fields
   extend ActiveSupport::Concern
 
-  INDEXES = %w[ most_discussed most_boosted newest oldest popped ]
+  INDEXES = %w[ most_discussed most_boosted most_stale newest oldest popped ]
 
   delegate :default_value?, to: :class
 

test/models/bubble/scorable_test.rb

@@ -50,7 +50,32 @@ class Bubble::ScorableTest < ActiveSupport::TestCase
       travel_back
       bubble_new.boost!
 
-      assert_equal [ bubble_new, bubble_mid, bubble_old ], Bubble.where(id: [ bubble_old, bubble_mid, bubble_new ]).ordered_by_activity
+      assert_equal %w[ new mid old ], Bubble.where(id: [ bubble_old, bubble_mid, bubble_new ]).ordered_by_activity.map(&:title)
+    end
+  end
+
+  test "items with old activity are more stale than those with none, or with new activity" do
+    with_current_user :kevin do
+      travel_to 20.days.ago
+      bubble_old = buckets(:writebook).bubbles.create! status: :published, title: "old"
+      bubble_new = buckets(:writebook).bubbles.create! status: :published, title: "new"
+      bubble_none = buckets(:writebook).bubbles.create! status: :published, title: "none"
+
+      bubble_old.boost!
+      bubble_old.boost!
+
+      travel_back
+      travel_to 2.days.ago
+      bubble_new.boost!
+      bubble_new.boost!
+
+      travel_back
+
+      assert_equal %w[ old new none ], Bubble.where(id: [ bubble_none, bubble_old, bubble_new ]).ordered_by_staleness.map(&:title)
+
+      bubble_old.boost!
+
+      assert_equal %w[ new old none ], Bubble.where(id: [ bubble_none, bubble_old, bubble_new ]).ordered_by_staleness.map(&:title)
     end
   end
 

test/models/filter_test.rb

@@ -110,6 +110,9 @@ class FilterTest < ActiveSupport::TestCase
 
     filters(:jz_assignments).update!(stages: [], assignees: [], tags: [], buckets: [ buckets(:writebook) ])
     assert_equal "Most discussed in Writebook", filters(:jz_assignments).summary
+
+    filters(:jz_assignments).update!(indexed_by: "most_stale")
+    assert_equal "Most stale in Writebook", filters(:jz_assignments).summary
   end
 
   test "params without a key-value pair" do