[Bug] Relation#union fails on virtual columns

spec/unit/relation/union_spec.rb

@@ -72,6 +72,94 @@
       end
     end
 
+    context "when the relations unioned have different names and virtual columns" do
+      let(:relation1) { relation.where(id: 1).select(:id, :name).select_append { [`NULL`.as(:title)] } }
+      let(:relation2) { tasks.select(:id).select_append { [`NULL`.as(:name)] }.select_append(:title) }
+
+      it "qualifies the table as the concatenated relation names" do
+        # this produces SQL like
+        # SELECT
+        #   `users__tasks`.`id`,
+        #   `users__tasks`.`name`,
+        #   NULL AS 'title'
+        # FROM
+        #   (
+        #     SELECT
+        #       *
+        #     FROM
+        #       (
+        #         SELECT
+        #           `users`.`id`,
+        #           `users`.`name`,
+        #           NULL AS 'title'
+        #         FROM
+        #           `users`
+        #         WHERE
+        #           (`id` = 1)
+        #         ORDER BY
+        #           `users`.`id`
+        #       ) AS 't1'
+        #     UNION
+        #     SELECT
+        #       *
+        #     FROM
+        #       (
+        #         SELECT
+        #           `tasks`.`id`,
+        #           NULL AS 'name',
+        #           `tasks`.`title`
+        #         FROM
+        #           `tasks`
+        #         ORDER BY
+        #           `tasks`.`id`
+        #       ) AS 't1'
+        #   ) AS 'users__tasks'
+        result = relation1.union(relation2)
+        # But in older versions of ROM and if using Sequel directly it produces:
+        # SELECT
+        #   *
+        # FROM
+        #   (
+        #     SELECT
+        #       *
+        #     FROM
+        #       (
+        #         SELECT
+        #           `users`.`id`,
+        #           `users`.`name`,
+        #           NULL AS 'title'
+        #         FROM
+        #           `users`
+        #         WHERE
+        #           (`id` = 1)
+        #         ORDER BY
+        #           `users`.`id`
+        #       ) AS 't1'
+        #     UNION
+        #     SELECT
+        #       *
+        #     FROM
+        #       (
+        #         SELECT
+        #           `tasks`.`id`,
+        #           NULL AS 'name',
+        #           `tasks`.`title`
+        #         FROM
+        #           `tasks`
+        #         ORDER BY
+        #           `tasks`.`id`
+        #       ) AS 't1'
+        #   ) AS 'users__tasks'
+        expect_to_have_qualified_name(result, :users__tasks)
+      end
+
+      it "has non-nil titles from the tasks relation" do
+        titles = relation1.union(relation2).to_a.map { |row| row.fetch(:title) }.compact
+        expect(titles).not_to be_empty
+      end
+    end
+
+
     def expect_to_have_qualified_name(rel, name)
       metas = rel.schema.map(&:meta)
       expect(metas).to all(include(qualified: name))