Merge branch 'tc-fix-rails5-subquery-selfjoin' into 'master'

Work around a bug in Rails 5, where LIMIT causes trouble

Closes #51729

See merge request gitlab-org/gitlab-ce!21879

lib/gitlab/database/subquery.rb

@@ -6,7 +6,15 @@ module Gitlab
       class << self
         def self_join(relation)
           t = relation.arel_table
-          t2 = relation.arel.as('t2')
+          t2 = if !Gitlab.rails5?
+                 relation.arel.as('t2')
+               else
+                 # Work around a bug in Rails 5, where LIMIT causes trouble
+                 # See https://gitlab.com/gitlab-org/gitlab-ce/issues/51729
+                 r = relation.limit(nil).arel
+                 r.take(relation.limit_value) if relation.limit_value
+                 r.as('t2')
+               end
 
           relation.unscoped.joins(t.join(t2).on(t[:id].eq(t2[:id])).join_sources.first)
         end