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, 1 insertions, 3 deletions

diff --git a/app/finders/snippets_finder.rb b/app/finders/snippets_finder.rb
index 715dffb972f..3528e4228b2 100644
--- a/app/finders/snippets_finder.rb
+++ b/app/finders/snippets_finder.rb
@@ -89,9 +89,7 @@ class SnippetsFinder < UnionFinder
 
     # We use a UNION here instead of OR clauses since this results in better
     # performance.
-    union = Gitlab::SQL::Union.new([authorized_projects.select('projects.id'), visible_projects.select('projects.id')])
-
-    Project.from("(#{union.to_sql}) AS #{Project.table_name}")
+    Project.from_union([authorized_projects, visible_projects])
   end
   # rubocop: enable CodeReuse/ActiveRecord