Revert "ActiveRecord::Base#<=> has been removed. Primary keys may not be in...

Revert "ActiveRecord::Base#<=> has been removed.  Primary keys may not be in order," -- will be replaced with a check to ensure that the keys used for comparison are integers, and only fail if they are not.

This reverts commit 6256734e.

Conflicts:
	activerecord/CHANGELOG.md

activerecord/CHANGELOG.md

@@ -262,18 +262,6 @@
 
     *kennyj*
 
-*   ActiveRecord::Base#<=> has been removed.  Primary keys may not be in order,
-    or even be numbers, so sorting by id doesn't make sense.  Please use `sort_by`
-    and specify the attribute you wish to sort with.  For example, change:
-
-      Post.all.to_a.sort
-
-    to:
-
-      Post.all.to_a.sort_by(&:id)
-
-    *Aaron Patterson*
-
 *   Fix: joins association, with defined in the scope block constraints by using several
     where constraints and at least of them is not `Arel::Nodes::Equality`,
     generates invalid SQL expression.

activerecord/lib/active_record/core.rb

@@ -310,6 +310,15 @@ def frozen?
       @attributes.frozen?
     end
 
+    # Allows sort on objects
+    def <=>(other_object)
+      if other_object.is_a?(self.class)
+        self.to_key <=> other_object.to_key
+      else
+        super
+      end
+    end
+
     # Returns +true+ if the record is read only. Records loaded through joins with piggy-back
     # attributes will be marked as read only since they cannot be saved.
     def readonly?

activerecord/test/cases/base_test.rb

@@ -554,6 +554,13 @@ def test_hashing
     assert_equal [ Topic.find(1) ], [ Topic.find(2).topic ] & [ Topic.find(1) ]
   end
 
+  def test_comparison
+    topic_1 = Topic.create!
+    topic_2 = Topic.create!
+
+    assert_equal [topic_2, topic_1].sort, [topic_1, topic_2]
+  end
+
   def test_create_without_prepared_statement
     topic = Topic.connection.unprepared_statement do
       Topic.create(:title => 'foo')