Handle null/not null properly. Fixes #8

lib/ransack/nodes/condition.rb

@@ -41,7 +41,7 @@ module Ransack
       end
 
       def valid_arity?
-        values.size <= 1 || predicate.compound || %w(in not_in).include?(predicate.name)
+        values.size <= 1 || predicate.wants_array
       end
 
       def attributes
@@ -188,11 +188,12 @@ module Ransack
       end
 
       def formatted_values_for_attribute(attr)
-        casted_values_for_attribute(attr).map do |val|
+        formatted = casted_values_for_attribute(attr).map do |val|
           val = attr.ransacker.formatter.call(val) if attr.ransacker && attr.ransacker.formatter
           val = predicate.format(val)
           val
         end
+        predicate.wants_array ? formatted : formatted.first
       end
 
       def default_type

lib/ransack/predicate.rb

@@ -1,6 +1,6 @@
 module Ransack
   class Predicate
-    attr_reader :name, :arel_predicate, :type, :formatter, :validator, :compound
+    attr_reader :name, :arel_predicate, :type, :formatter, :validator, :compound, :wants_array
 
     class << self
 
@@ -41,6 +41,7 @@ module Ransack
       @formatter = opts[:formatter]
       @validator = opts[:validator] || lambda { |v| v.present? }
       @compound = opts[:compound]
+      @wants_array = @compound || ['in', 'not_in'].include?(@arel_predicate)
     end
 
     def eql?(other)