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/database/grant.rb b/lib/gitlab/database/grant.rb
index d32837f5793..7d334a79009 100644
--- a/lib/gitlab/database/grant.rb
+++ b/lib/gitlab/database/grant.rb
@@ -2,6 +2,8 @@ module Gitlab
   module Database
     # Model that can be used for querying permissions of a SQL user.
     class Grant < ActiveRecord::Base
+      include FromUnion
+
       self.table_name =
         if Database.postgresql?
           'information_schema.role_table_grants'
@@ -42,9 +44,7 @@ module Gitlab
               .where("GRANTEE = CONCAT('\\'', REPLACE(CURRENT_USER(), '@', '\\'@\\''), '\\'')")
           ]
 
-          union = SQL::Union.new(queries).to_sql
-
-          Grant.from("(#{union}) privs").any?
+          Grant.from_union(queries, alias_as: 'privs').any?
         end
       end
     end