Override respond_to? in ActiveRecord::Relation to go with

method_missing.

activerecord/lib/active_record/relation.rb

@@ -73,6 +73,14 @@ module ActiveRecord
       end
     end
 
+    def respond_to?(method)
+      if @relation.respond_to?(method) || Array.instance_methods.include?(method.to_s)
+        true
+      else
+        super
+      end
+    end
+
     private
       def method_missing(method, *args, &block)
         if @relation.respond_to?(method)

activerecord/test/cases/relations_test.rb

@@ -71,5 +71,13 @@ class RelationTest < ActiveRecord::TestCase
     ).to_a
     assert_equal 1, person_with_reader_and_post.size
   end
+
+  def test_relation_responds_to_delegated_methods
+    relation = Topic.all
+
+    ["map", "uniq", "sort", "insert", "delete", "update"].each do |method|
+      assert relation.respond_to?(method)
+    end
+  end
 end