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, 5 insertions, 3 deletions

diff --git a/app/models/project_authorization.rb b/app/models/project_authorization.rb
index 746bb4584c9..2c590008db2 100644
--- a/app/models/project_authorization.rb
+++ b/app/models/project_authorization.rb
@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 class ProjectAuthorization < ActiveRecord::Base
+  include FromUnion
+
   belongs_to :user
   belongs_to :project
 
@@ -8,9 +10,9 @@ class ProjectAuthorization < ActiveRecord::Base
   validates :access_level, inclusion: { in: Gitlab::Access.all_values }, presence: true
   validates :user, uniqueness: { scope: [:project, :access_level] }, presence: true
 
-  def self.select_from_union(union)
-    select(['project_id', 'MAX(access_level) AS access_level'])
-      .from("(#{union.to_sql}) #{ProjectAuthorization.table_name}")
+  def self.select_from_union(relations)
+    from_union(relations)
+      .select(['project_id', 'MAX(access_level) AS access_level'])
       .group(:project_id)
   end