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

activerecord/CHANGELOG.md

+*   Add a warning for enum elements with 'not_' prefix.
+
+        class Foo
+          enum status: [:sent, :not_sent]
+        end
+
+    *Edu Depetris*
+
 *   Loading the schema for a model that has no `table_name` raises a `TableNotSpecified` error.
 
     *Guilherme Mansur*, *Eugene Kenny*

activerecord/lib/active_record/enum.rb

@@ -200,6 +200,8 @@ def enum(definitions)
             # scope :active, -> { where(status: 0) }
             # scope :not_active, -> { where.not(status: 0) }
             if enum_scopes != false
+              klass.send(:detect_negative_condition!, value_method_name)
+
               klass.send(:detect_enum_conflict!, name, value_method_name, true)
               klass.scope value_method_name, -> { where(attr => value) }
 
@@ -261,5 +263,12 @@ def raise_conflict_error(enum_name, method_name, type: "instance", source: "Acti
           source: source
         }
       end
+
+      def detect_negative_condition!(method_name)
+        if method_name.start_with?("not_") && logger
+          logger.warn "An enum element in #{self.name} uses the prefix 'not_'." \
+            " This will cause a conflict with auto generated negative scopes."
+        end
+      end
   end
 end

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 @@ def self.name; "Book"; end
 
     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