Try next values in array only.

If we can't find a valid next value outside of each item
in the array, throw a custom exception.

lib/shoulda/matchers/active_model.rb

@@ -13,6 +13,7 @@ require 'shoulda/matchers/active_model/validate_acceptance_of_matcher'
 require 'shoulda/matchers/active_model/validate_confirmation_of_matcher'
 require 'shoulda/matchers/active_model/validate_numericality_of_matcher'
 require 'shoulda/matchers/active_model/allow_mass_assignment_of_matcher'
+require 'shoulda/matchers/active_model/errors'
 
 
 module Shoulda

lib/shoulda/matchers/active_model/ensure_inclusion_of_matcher.rb

@@ -83,7 +83,7 @@ module Shoulda # :nodoc:
               disallows_higher_value &&
               allows_maximum_value
           elsif @array
-            if allows_all_values_in_array? && allows_blank_value? && allows_nil_value? && disallows_outside_values?
+            if allows_all_values_in_array? && allows_blank_value? && allows_nil_value? && disallows_value_outside_of_array?
               true
             else
               @failure_message = "#{@array} doesn't match array in validation"
@@ -137,16 +137,22 @@ module Shoulda # :nodoc:
           allows_value_of(@maximum, @high_message)
         end
 
-        def disallows_outside_values?
-          disallows_value_of(value_outside_of_array(@array))
+        def disallows_value_outside_of_array?
+          if value_outside_of_array
+            disallows_value_of(value_outside_of_array)
+          else
+            raise CouldNotDetermineValueOutsideOfArray
+          end
         end
 
-        def value_outside_of_array(array)
-          not_in_array = array.last.next
-          while array.include?(not_in_array)
-            not_in_array.next!
+        def value_outside_of_array
+          found = @array.detect do |item|
+            !@array.include?(item.next)
+          end
+
+          if found
+            found.next
           end
-          not_in_array
         end
       end
     end

lib/shoulda/matchers/active_model/errors.rb

@@ -0,0 +1,7 @@
+module Shoulda # :nodoc:
+  module Matchers
+    module ActiveModel # :nodoc:
+      class CouldNotDetermineValueOutsideOfArray < RuntimeError; end
+    end
+  end
+end

spec/shoulda/active_model/ensure_inclusion_of_matcher_spec.rb

@@ -12,6 +12,14 @@ describe Shoulda::Matchers::ActiveModel::EnsureInclusionOfMatcher do
     end
   end
 
+  context "where we cannot determine a value outside the array" do
+    it "should raise a custom exception" do
+      @model = define_model(:example, :attr => :string).new
+
+      expect { @model.should ensure_inclusion_of(:attr).in_array([""]) }.to raise_error Shoulda::Matchers::ActiveModel::CouldNotDetermineValueOutsideOfArray
+    end
+  end
+
   context "an attribute which must be included in a range" do
     before do
       @model = define_model(:example, :attr => :integer) do