From 5dc650aa9bb3cbb9d78a731b5c9788d96cc32ea4 Mon Sep 17 00:00:00 2001
From: Daniel Gollahon <dgollahon@plaid.com>
Date: Sat, 3 Jun 2023 08:01:26 -0700
Subject: [PATCH] Relation#union fails on virtual columns

- When merging relations with virtual columns the qualified projection (added in #334) causes the virtual column to be moved into the outer select. This means that column will always be hardcoded to `NULL` instead of selecting from the result of the union. This is a [not-that-esoteric technique with unions](https://stackoverflow.com/a/39766372). This also worked in ROM prior to #334.

This PR is really an issue/bug report just in the form of a failing test. I don't have a good idea as to what the right solutin is but I figured it would be helpful to at least have a very concrete demonstration of the issue.
---
 spec/unit/relation/union_spec.rb | 88 ++++++++++++++++++++++++++++++++
 1 file changed, 88 insertions(+)

diff --git a/spec/unit/relation/union_spec.rb b/spec/unit/relation/union_spec.rb
index d1307c63..dcf021c0 100644
--- a/spec/unit/relation/union_spec.rb
+++ b/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))