Add a warning for enum elements with 'not_' prefix.

When a enum element contains the prefix 'not_'. I warns to users
to be aware of this new feature.

Example code:
    class Foo < ActiveRecord::Base
      enum status: [:sent, :not_sent]
    end

1 files changed, 22 insertions, 0 deletions

diff --git a/activerecord/test/cases/enum_test.rb b/activerecord/test/cases/enum_test.rb
index ae0ce195b3..8673a99c45 100644
--- a/activerecord/test/cases/enum_test.rb
+++ b/activerecord/test/cases/enum_test.rb
@@ -3,6 +3,7 @@
 require "cases/helper"
 require "models/author"
 require "models/book"
+require "active_support/log_subscriber/test_helper"
 
 class EnumTest < ActiveRecord::TestCase
   fixtures :books, :authors, :author_addresses
@@ -565,4 +566,25 @@ class EnumTest < ActiveRecord::TestCase
 
     assert_raises(NoMethodError) { klass.proposed }
   end
+
+  test "enums with a negative condition log a warning" do
+    old_logger = ActiveRecord::Base.logger
+    logger = ActiveSupport::LogSubscriber::TestHelper::MockLogger.new
+
+    ActiveRecord::Base.logger = logger
+
+    expected_message = "An enum element in Book uses the prefix 'not_'."\
+      " This will cause a conflict with auto generated negative scopes."
+
+    Class.new(ActiveRecord::Base) do
+      def self.name
+        "Book"
+      end
+      enum status: [:sent, :not_sent]
+    end
+
+    assert_match(expected_message, logger.logged(:warn).first)
+  ensure
+    ActiveRecord::Base.logger = old_logger
+  end
 end