5 files changed, 93 insertions(+), 5 deletions(-)

M app/services/fetch_link_card_service.rb
A db/post_migrate/20230803082451_add_unique_index_on_preview_cards_statuses.rb
A db/post_migrate/20230803112520_add_primary_key_to_preview_cards_statuses_join_table.rb
M db/schema.rb
M lib/tasks/tests.rake

@@ 61,9 61,13 @@ class FetchLinkCardService < BaseService
   end
 
   def attach_card
-    @status.preview_cards << @card
-    Rails.cache.delete(@status)
-    Trends.links.register(@status)
+    with_redis_lock("attach_card:#{@status.id}") do
+      return if @status.preview_cards.any?
+
+      @status.preview_cards << @card
+      Rails.cache.delete(@status)
+      Trends.links.register(@status)
+    end
   end
 
   def parse_urls

@@ 0,0 1,39 @@
+# frozen_string_literal: true
+
+class AddUniqueIndexOnPreviewCardsStatuses < ActiveRecord::Migration[6.1]
+  disable_ddl_transaction!
+
+  def up
+    add_index :preview_cards_statuses, [:status_id, :preview_card_id], name: :preview_cards_statuses_pkey, algorithm: :concurrently, unique: true
+  rescue ActiveRecord::RecordNotUnique
+    deduplicate_and_reindex!
+  end
+
+  def down
+    remove_index :preview_cards_statuses, name: :preview_cards_statuses_pkey
+  end
+
+  private
+
+  def deduplicate_and_reindex!
+    deduplicate_preview_cards!
+
+    safety_assured { execute 'REINDEX INDEX preview_cards_statuses_pkey' }
+  rescue ActiveRecord::RecordNotUnique
+    retry
+  end
+
+  def deduplicate_preview_cards!
+    # Statuses should have only one preview card at most, even if that's not the database
+    # constraint we will end up with
+    duplicate_ids = select_all('SELECT status_id FROM preview_cards_statuses GROUP BY status_id HAVING count(*) > 1;').rows
+
+    duplicate_ids.each_slice(1000) do |ids|
+      # This one is tricky: since we don't have primary keys to keep only one record,
+      # use the physical `ctid`
+      safety_assured do
+        execute "DELETE FROM preview_cards_statuses p WHERE p.status_id IN (#{ids.join(', ')}) AND p.ctid NOT IN (SELECT q.ctid FROM preview_cards_statuses q WHERE q.status_id = p.status_id LIMIT 1)"
+      end
+    end
+  end
+end

@@ 0,0 1,20 @@
+# frozen_string_literal: true
+
+class AddPrimaryKeyToPreviewCardsStatusesJoinTable < ActiveRecord::Migration[6.1]
+  disable_ddl_transaction!
+
+  def up
+    safety_assured do
+      execute 'ALTER TABLE preview_cards_statuses ADD PRIMARY KEY USING INDEX preview_cards_statuses_pkey'
+    end
+  end
+
+  def down
+    safety_assured do
+      # I have found no way to demote the primary key to an index, instead, re-create the index
+      execute 'CREATE UNIQUE INDEX CONCURRENTLY preview_cards_statuses_pkey_tmp ON preview_cards_statuses (status_id, preview_card_id)'
+      execute 'ALTER TABLE preview_cards_statuses DROP CONSTRAINT preview_cards_statuses_pkey'
+      execute 'ALTER INDEX preview_cards_statuses_pkey_tmp RENAME TO preview_cards_statuses_pkey'
+    end
+  end
+end

@@ 10,7 10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.0].define(version: 2023_07_24_160715) do
+ActiveRecord::Schema[7.0].define(version: 2023_08_03_112520) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
 


@@ 805,7 805,7 @@ ActiveRecord::Schema[7.0].define(version: 2023_07_24_160715) do
     t.index ["url"], name: "index_preview_cards_on_url", unique: true
   end
 
-  create_table "preview_cards_statuses", id: false, force: :cascade do |t|
+  create_table "preview_cards_statuses", primary_key: ["status_id", "preview_card_id"], force: :cascade do |t|
     t.bigint "preview_card_id", null: false
     t.bigint "status_id", null: false
     t.index ["status_id", "preview_card_id"], name: "index_preview_cards_statuses_on_status_id_and_preview_card_id"

@@ 63,6 63,11 @@ namespace :tests do
         puts 'Account domains not properly normalized'
         exit(1)
       end
+
+      unless Status.find(12).preview_cards.pluck(:url) == ['https://joinmastodon.org/']
+        puts 'Preview cards not deduplicated as expected'
+        exit(1)
+      end
     end
 
     desc 'Populate the database with test data for 2.4.3'


@@ 238,6 243,11 @@ namespace :tests do
           (10, 2, '@admin hey!', NULL, 1, 3, now(), now()),
           (11, 1, '@user hey!', 10, 1, 3, now(), now());
 
+        INSERT INTO "statuses"
+          (id, account_id, text, created_at, updated_at)
+        VALUES
+          (12, 1, 'check out https://joinmastodon.org/', now(), now());
+
         -- mentions (from previous statuses)
 
         INSERT INTO "mentions"


@@ 326,6 336,21 @@ namespace :tests do
           (1, 6, 2, 'Follow', 2, now(), now()),
           (2, 2, 1, 'Mention', 4, now(), now()),
           (3, 1, 2, 'Mention', 5, now(), now());
+
+        -- preview cards
+
+        INSERT INTO "preview_cards"
+          (id, url, title, created_at, updated_at)
+        VALUES
+          (1, 'https://joinmastodon.org/', 'Mastodon - Decentralized social media', now(), now());
+
+        -- many-to-many association between preview cards and statuses
+
+        INSERT INTO "preview_cards_statuses"
+          (status_id, preview_card_id)
+        VALUES
+          (12, 1),
+          (12, 1);
       SQL
     end
   end