Remove thing_type and thing_id columns from settings table (#31971)

db/post_migrate/20241205135925_remove_legacy_user_settings_columns.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+class RemoveLegacyUserSettingsColumns < ActiveRecord::Migration[7.2]
+  disable_ddl_transaction!
+
+  def up
+    # In normal usage this should not find anything to delete
+    # Deletion here is already done in RemoveLegacyUserSettingsData migration
+    # and no data like this should be created from app at this point
+    # Deleting again out of caution
+    connection.execute(<<~SQL.squish)
+      DELETE FROM settings
+      WHERE
+        thing_type IS NOT NULL
+        AND thing_id IS NOT NULL
+    SQL
+
+    add_index :settings, :var, unique: true, algorithm: :concurrently
+    remove_index :settings, [:thing_type, :thing_id, :var], name: :index_settings_on_thing_type_and_thing_id_and_var, unique: true
+
+    safety_assured do
+      remove_column :settings, :thing_type, :string
+      remove_column :settings, :thing_id, :bigint
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end