Fix follow recommendation biased towards older accounts (#17126)

app/workers/scheduler/follow_recommendations_scheduler.rb

@@ -16,12 +16,12 @@ class Scheduler::FollowRecommendationsScheduler
     AccountSummary.refresh
     FollowRecommendation.refresh
 
-    fallback_recommendations = FollowRecommendation.limit(SET_SIZE).index_by(&:account_id)
+    fallback_recommendations = FollowRecommendation.order(rank: :desc).limit(SET_SIZE).index_by(&:account_id)
 
     I18n.available_locales.each do |locale|
       recommendations = begin
         if AccountSummary.safe.filtered.localized(locale).exists? # We can skip the work if no accounts with that language exist
-          FollowRecommendation.localized(locale).limit(SET_SIZE).index_by(&:account_id)
+          FollowRecommendation.localized(locale).order(rank: :desc).limit(SET_SIZE).index_by(&:account_id)
         else
           {}
         end

db/migrate/20211213040746_update_account_summaries_to_version_2.rb

@@ -0,0 +1,24 @@
+class UpdateAccountSummariesToVersion2 < ActiveRecord::Migration[6.1]
+  def up
+    reapplication_follow_recommendations_v2 do
+      drop_view :account_summaries, materialized: true
+      create_view :account_summaries, version: 2, materialized: { no_data: true }
+      safety_assured { add_index :account_summaries, :account_id, unique: true }
+    end
+  end
+
+  def down
+    reapplication_follow_recommendations_v2 do
+      drop_view :account_summaries, materialized: true
+      create_view :account_summaries, version: 1, materialized: { no_data: true }
+      safety_assured { add_index :account_summaries, :account_id, unique: true }
+    end
+  end
+
+  def reapplication_follow_recommendations_v2
+    drop_view :follow_recommendations, materialized: true
+    yield
+    create_view :follow_recommendations, version: 2, materialized: { no_data: true }
+    safety_assured { add_index :follow_recommendations, :account_id, unique: true }
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2021_11_26_000907) do
+ActiveRecord::Schema.define(version: 2021_12_13_040746) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -1129,7 +1129,7 @@ ActiveRecord::Schema.define(version: 2021_11_26_000907) do
               statuses.language,
               statuses.sensitive
              FROM statuses
-            WHERE ((statuses.account_id = accounts.id) AND (statuses.deleted_at IS NULL))
+            WHERE ((statuses.account_id = accounts.id) AND (statuses.deleted_at IS NULL) AND (statuses.reblog_of_id IS NULL))
             ORDER BY statuses.id DESC
            LIMIT 20) t0)
     WHERE ((accounts.suspended_at IS NULL) AND (accounts.silenced_at IS NULL) AND (accounts.moved_to_account_id IS NULL) AND (accounts.discoverable = true) AND (accounts.locked = false))

db/views/account_summaries_v02.sql

@@ -0,0 +1,23 @@
+SELECT
+  accounts.id AS account_id,
+  mode() WITHIN GROUP (ORDER BY language ASC) AS language,
+  mode() WITHIN GROUP (ORDER BY sensitive ASC) AS sensitive
+FROM accounts
+CROSS JOIN LATERAL (
+  SELECT
+    statuses.account_id,
+    statuses.language,
+    statuses.sensitive
+  FROM statuses
+  WHERE statuses.account_id = accounts.id
+    AND statuses.deleted_at IS NULL
+    AND statuses.reblog_of_id IS NULL
+  ORDER BY statuses.id DESC
+  LIMIT 20
+) t0
+WHERE accounts.suspended_at IS NULL
+  AND accounts.silenced_at IS NULL
+  AND accounts.moved_to_account_id IS NULL
+  AND accounts.discoverable = 't'
+  AND accounts.locked = 'f'
+GROUP BY accounts.id