Do not in place modify what table_name returns

activemodel/lib/active_model/attribute_methods.rb

@@ -107,7 +107,7 @@ module ActiveModel
           sing.send :define_method, name, &block
         else
           value = value.to_s if value
-          sing.send(:define_method, name) { value && value.dup }
+          sing.send(:define_method, name) { value }
         end
       end
 

activemodel/test/cases/attribute_methods_test.rb

@@ -9,10 +9,6 @@ class ModelWithAttributes
     define_method(:bar) do
       'original bar'
     end
-
-    define_method(:zomg) do
-      'original zomg'
-    end
   end
 
   def attributes
@@ -102,13 +98,6 @@ class AttributeMethodsTest < ActiveModel::TestCase
     assert_equal "value of foo bar", ModelWithAttributesWithSpaces.new.send(:'foo bar')
   end
 
-  def test_defined_methods_always_return_duped_string
-    ModelWithAttributes.define_attr_method(:zomg, 'lol')
-    assert_equal 'lol', ModelWithAttributes.zomg
-    ModelWithAttributes.zomg << 'bbq'
-    assert_equal 'lol', ModelWithAttributes.zomg
-  end
-
   test '#define_attr_method generates attribute method' do
     ModelWithAttributes.define_attr_method(:bar, 'bar')
 

activerecord/lib/active_record/base.rb

@@ -995,7 +995,7 @@ module ActiveRecord #:nodoc:
             if parent < ActiveRecord::Base && !parent.abstract_class?
               contained = parent.table_name
               contained = contained.singularize if parent.pluralize_table_names
-              contained << '_'
+              contained += '_'
             end
             "#{full_table_name_prefix}#{contained}#{undecorated_table_name(name)}#{table_name_suffix}"
           else