Added FromUnion to easily select from a UNION

This commit adds the module `FromUnion`, which provides the class method
`from_union`. This simplifies the process of selecting data from the
result of a UNION, and reduces the likelihood of making mistakes. As a
result, instead of this:

    union = Gitlab::SQL::Union.new([foo, bar])

    Foo.from("(#{union.to_sql}) #{Foo.table_name}")

We can now write this instead:

    Foo.from_union([foo, bar])

This commit also includes some changes to make this new setup work
properly. For example, a bug in Rails 4
(https://github.com/rails/rails/issues/24193) would break the use of
`from("sub-query-here").includes(:relation)` in certain cases. There was
also a CI query which appeared to repeat a lot of conditions from an
outer query on an inner query, which isn't necessary.

Finally, we include a RuboCop cop to ensure developers use this new
module, instead of using Gitlab::SQL::Union directly.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/51307

1 files changed, 7 insertions, 7 deletions

diff --git a/app/services/labels/transfer_service.rb b/app/services/labels/transfer_service.rb
index aec0282b31b..52360f775dc 100644
--- a/app/services/labels/transfer_service.rb
+++ b/app/services/labels/transfer_service.rb
@@ -34,13 +34,13 @@ module Labels
 
     # rubocop: disable CodeReuse/ActiveRecord
     def labels_to_transfer
-      label_ids = []
-      label_ids << group_labels_applied_to_issues.select(:id)
-      label_ids << group_labels_applied_to_merge_requests.select(:id)
-
-      union = Gitlab::SQL::Union.new(label_ids)
-
-      Label.where("labels.id IN (#{union.to_sql})").reorder(nil).uniq # rubocop:disable GitlabSecurity/SqlInjection
+      Label
+        .from_union([
+          group_labels_applied_to_issues,
+          group_labels_applied_to_merge_requests
+        ])
+        .reorder(nil)
+        .uniq
     end
     # rubocop: enable CodeReuse/ActiveRecord