Allow writing unknown attributes, but with a deprecation warning. Closes #4583.

activerecord/lib/active_record/attribute_methods/write.rb

@@ -28,11 +28,14 @@ module ActiveRecord
         @attributes_cache.delete(attr_name)
         column = column_for_attribute(attr_name)
 
-        if column || @attributes.has_key?(attr_name)
-          @attributes[attr_name] = type_cast_attribute_for_write(column, value)
-        else
-          raise ActiveModel::MissingAttributeError, "can't write unknown attribute `#{attr_name}'"
+        unless column || @attributes.has_key?(attr_name)
+          ActiveSupport::Deprecation.warn(
+            "You're trying to create an attribute `#{attr_name}'. Writing arbitrary " \
+            "attributes on a model is deprecated. Please just use `attr_writer` etc."
+          )
         end
+
+        @attributes[attr_name] = type_cast_attribute_for_write(column, value)
       end
       alias_method :raw_write_attribute, :write_attribute
 

activerecord/test/cases/attribute_methods_test.rb

@@ -780,6 +780,13 @@ class AttributeMethodsTest < ActiveRecord::TestCase
     assert_deprecated { klass.define_attribute_methods }
   end
 
+  def test_setting_new_attributes_deprecated
+    t = Topic.new
+    assert_deprecated { t[:foo] = "bar" }
+    assert_equal "bar", t.foo
+    assert_equal "bar", t[:foo]
+  end
+
   private
   def cached_columns
     @cached_columns ||= time_related_columns_on_topic.map(&:name)