use quoted id of single AR::Base objects in predicates

activerecord/lib/active_record/relation/predicate_builder.rb

@@ -25,6 +25,8 @@ def self.build_from_hash(engine, attributes, default_table)
             attribute.in(values)
           when Range, Arel::Relation
             attribute.in(value)
+          when ActiveRecord::Base
+            attribute.eq(value.quoted_id)
           else
             attribute.eq(value)
           end

activerecord/test/cases/relations_test.rb

@@ -426,6 +426,12 @@ def test_find_in_empty_array
     assert_blank authors.all
   end
 
+  def test_where_with_ar_object
+    author = Author.first
+    authors = Author.scoped.where(:id => author)
+    assert_equal 1, authors.all.length
+  end
+
   def test_exists
     davids = Author.where(:name => 'David')
     assert davids.exists?