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

2 files changed, 2 insertions, 6 deletions

diff --git a/lib/gitlab/project_authorizations/with_nested_groups.rb b/lib/gitlab/project_authorizations/with_nested_groups.rb
index e3da1634fa5..448c3f3a7d8 100644
--- a/lib/gitlab/project_authorizations/with_nested_groups.rb
+++ b/lib/gitlab/project_authorizations/with_nested_groups.rb
@@ -49,13 +49,11 @@ module Gitlab
             .where('p_ns.share_with_group_lock IS FALSE')
         ]
 
-        union = Gitlab::SQL::Union.new(relations)
-
         ProjectAuthorization
           .unscoped
           .with
           .recursive(cte.to_arel)
-          .select_from_union(union)
+          .select_from_union(relations)
       end
 
       private
diff --git a/lib/gitlab/project_authorizations/without_nested_groups.rb b/lib/gitlab/project_authorizations/without_nested_groups.rb
index 7d0c00c7f36..ed2287dcc7e 100644
--- a/lib/gitlab/project_authorizations/without_nested_groups.rb
+++ b/lib/gitlab/project_authorizations/without_nested_groups.rb
@@ -24,11 +24,9 @@ module Gitlab
           user.groups.joins(:shared_projects).select_for_project_authorization
         ]
 
-        union = Gitlab::SQL::Union.new(relations)
-
         ProjectAuthorization
           .unscoped
-          .select_from_union(union)
+          .select_from_union(relations)
       end
     end
   end