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, 3 insertions, 3 deletions

diff --git a/lib/gitlab/user_extractor.rb b/lib/gitlab/user_extractor.rb
index 09652a4fb5e..bd0d24e4369 100644
--- a/lib/gitlab/user_extractor.rb
+++ b/lib/gitlab/user_extractor.rb
@@ -18,7 +18,7 @@ module Gitlab
     def users
       return User.none unless @text.present?
 
-      @users ||= User.from("(#{union.to_sql}) users")
+      @users ||= User.from_union(union_relations)
     end
     # rubocop: enable CodeReuse/ActiveRecord
 
@@ -43,13 +43,13 @@ module Gitlab
 
     private
 
-    def union
+    def union_relations
       relations = []
 
       relations << User.by_any_email(emails) if emails.any?
       relations << User.by_username(usernames) if usernames.any?
 
-      Gitlab::SQL::Union.new(relations)
+      relations
     end
   end
 end