Ensure that the article_id is set for every article_price

This addresses the TODO from dc92de771e.

db/migrate/20181201000305_ensure_article_for_article_price.rb

@@ -0,0 +1,20 @@
+class EnsureArticleForArticlePrice < ActiveRecord::Migration
+  class ArticlePrice < ActiveRecord::Base; end
+
+  def change
+    reversible do |dir|
+      dir.up do
+        execute <<-SQL
+          UPDATE article_prices SET article_id = (
+            SELECT article_id FROM order_articles
+            WHERE article_price_id = article_prices.id
+          )
+          WHERE article_id IS NULL
+        SQL
+        ArticlePrice.where(article_id: nil).destroy_all
+      end
+    end
+
+    change_column_null :article_prices, :article_id, false
+  end
+end