Don't enable validations when passing false hash values to ActiveModel.validates

Passing a falsey option value for a validator currently causes that validator to
be enabled, just like "true":

    ActiveModel.validates :foo, :presence => false

This is rather counterintuitive, and makes it inconvenient to wrap `validates` in
methods which may conditionally enable different validators.

As an example, one is currently forced to write:

      def has_slug(source_field, options={:unique => true})
        slugger = Proc.new { |r| r[:slug] = self.class.sluggify(r[source_field]) if r[:slug].blank? }
        before_validation slugger
        validations = { :presence => true, :slug => true }
        if options[:unique]
          validations[:uniqueness] = true
        end
        validates :slug, validations
      end

because the following reasonable-looking alternative fails to work as expected:

      def has_slug(source_field, options={:unique => true})
        slugger = Proc.new { |r| r[:slug] = self.class.sluggify(r[source_field]) if r[:slug].blank? }
        before_validation slugger
        validates :slug, :presence => true, :slug => true, :uniqueness => options[:unique]
      end

(This commit includes a test, and all activemodel and activerecord tests pass as before.)

1 files changed, 5 insertions, 0 deletions

diff --git a/activemodel/test/cases/validations_test.rb b/activemodel/test/cases/validations_test.rb
index 1f5023bf76..8ea9745fbf 100644
--- a/activemodel/test/cases/validations_test.rb
+++ b/activemodel/test/cases/validations_test.rb
@@ -330,6 +330,11 @@ class ValidationsTest < ActiveModel::TestCase
     end
   end
 
+  def test_validates_with_false_hash_value
+    Topic.validates :title,  :presence => false
+    assert Topic.new.valid?
+  end
+
   def test_strict_validation_error_message
     Topic.validates :title, :strict => true, :presence => true