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)

activerecord/CHANGELOG.md

@@ -1,3 +1,13 @@
+*   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)
+
 *   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

@@ -308,15 +308,6 @@ module ActiveRecord
       @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

@@ -558,13 +558,6 @@ class BasicsTest < ActiveRecord::TestCase
     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')