DEV: Delete upload references upon deleting draft (#22851)

We currently are accumulating orphaned upload references whenever drafts are deleted.

This change deals with future cases by adding a dependent strategy of delete_all on the Draft#upload_references association. (We don't really need destroy strategy here, since UploadReference is a simple data bag and there are no validations or callbacks on the model.)

It deals with existing cases through a migration that deletes all existing, orphaned draft upload references.

app/models/draft.rb

@@ -7,6 +7,8 @@ class Draft < ActiveRecord::Base
 
   belongs_to :user
 
+  has_many :upload_references, as: :target, dependent: :delete_all
+
   after_commit :update_draft_count, on: %i[create destroy]
 
   class OutOfSequence < StandardError

db/migrate/20230728055813_delete_orphaned_draft_upload_references.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+class DeleteOrphanedDraftUploadReferences < ActiveRecord::Migration[7.0]
+  def up
+    execute <<~SQL
+      DELETE
+      FROM
+        "upload_references"
+      WHERE
+        "upload_references"."target_type" = 'Draft' AND
+        "upload_references"."target_id" NOT IN (
+          SELECT "drafts"."id" FROM "drafts"
+        )
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end

spec/models/draft_spec.rb

@@ -5,6 +5,8 @@ RSpec.describe Draft do
 
   fab!(:post) { Fabricate(:post) }
 
+  it { is_expected.to have_many(:upload_references).dependent(:delete_all) }
+
   describe "system user" do
     it "can not set drafts" do
       # fake a sequence