Put back Relation#join method as a delegate to Array

This is a regression 4.0 -> 4.1 fix.
In 4.1.0 Relation#join is delegated to Arel#SelectManager.
In 4.0 series it is delegated to Array#join

This patch puts back the behaviour of 4.0

3 files changed, 9 insertions, 1 deletions

diff --git a/activerecord/test/cases/relation/delegation_test.rb b/activerecord/test/cases/relation/delegation_test.rb
index 9b2bfed039..29c9d0e2af 100644
--- a/activerecord/test/cases/relation/delegation_test.rb
+++ b/activerecord/test/cases/relation/delegation_test.rb
@@ -32,7 +32,7 @@ module ActiveRecord
       :exclude?, :find_all, :flat_map, :group_by, :include?, :length,
       :map, :none?, :one?, :partition, :reject, :reverse,
       :sample, :second, :sort, :sort_by, :third,
-      :to_ary, :to_set, :to_xml, :to_yaml
+      :to_ary, :to_set, :to_xml, :to_yaml, :join
     ]
 
     ARRAY_DELEGATES.each do |method|
diff --git a/activerecord/test/cases/relations_test.rb b/activerecord/test/cases/relations_test.rb
index 68e62934c1..6a880c6680 100644
--- a/activerecord/test/cases/relations_test.rb
+++ b/activerecord/test/cases/relations_test.rb
@@ -1677,4 +1677,8 @@ class RelationTest < ActiveRecord::TestCase
     merged = left.merge(right)
     assert_equal post, merged.first
   end
+
+  def test_relation_join_method
+    assert_equal 'Thank you for the welcome,Thank you again for the welcome', Post.first.comments.join(",")
+  end
 end
diff --git a/activerecord/test/models/comment.rb b/activerecord/test/models/comment.rb
index ede5fbd0c6..f82df417ce 100644
--- a/activerecord/test/models/comment.rb
+++ b/activerecord/test/models/comment.rb
@@ -26,6 +26,10 @@ class Comment < ActiveRecord::Base
     all
   end
   scope :all_as_scope, -> { all }
+
+  def to_s
+    body
+  end
 end
 
 class SpecialComment < Comment