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, 6 insertions, 8 deletions

diff --git a/app/models/project.rb b/app/models/project.rb
index c37915e111f..9e4c7f7a2d0 100644
--- a/app/models/project.rb
+++ b/app/models/project.rb
@@ -29,6 +29,7 @@ class Project < ActiveRecord::Base
   include BatchDestroyDependentAssociations
   include FeatureGate
   include OptionallySearch
+  include FromUnion
   extend Gitlab::Cache::RequestCache
 
   extend Gitlab::ConfigHelper
@@ -1493,8 +1494,7 @@ class Project < ActiveRecord::Base
   end
 
   def all_runners
-    union = Gitlab::SQL::Union.new([runners, group_runners, shared_runners])
-    Ci::Runner.from("(#{union.to_sql}) ci_runners")
+    Ci::Runner.from_union([runners, group_runners, shared_runners])
   end
 
   def active_runners
@@ -2022,12 +2022,10 @@ class Project < ActiveRecord::Base
   def badges
     return project_badges unless group
 
-    group_badges_rel = GroupBadge.where(group: group.self_and_ancestors)
-
-    union = Gitlab::SQL::Union.new([project_badges.select(:id),
-                                    group_badges_rel.select(:id)])
-
-    Badge.where("id IN (#{union.to_sql})") # rubocop:disable GitlabSecurity/SqlInjection
+    Badge.from_union([
+      project_badges,
+      GroupBadge.where(group: group.self_and_ancestors)
+    ])
   end
 
   def merge_requests_allowing_push_to_user(user)