Fix media attachments enumeration (#14254)

* Fix media attachment enumeration

* Switch media_attachments id to snowflake ids

Co-authored-by: Thibaut Girka <thib@sitedethib.com>

db/migrate/20200622213645_media_attachment_ids_to_timestamp_ids.rb

@@ -0,0 +1,17 @@
+class MediaAttachmentIdsToTimestampIds < ActiveRecord::Migration[5.1]
+  def up
+    # Set up the media_attachments.id column to use our timestamp-based IDs.
+    safety_assured do
+      execute("ALTER TABLE media_attachments ALTER COLUMN id SET DEFAULT timestamp_id('media_attachments')")
+    end
+
+    # Make sure we have a sequence to use.
+    Mastodon::Snowflake.ensure_id_sequences_exist
+  end
+
+  def down
+    execute("LOCK media_attachments")
+    execute("SELECT setval('media_attachments_id_seq', (SELECT MAX(id) FROM media_attachments))")
+    execute("ALTER TABLE media_attachments ALTER COLUMN id SET DEFAULT nextval('media_attachments_id_seq')")
+  end
+end