Make implicit order column configurable

When calling ordered finder methods such as +first+ or +last+ without an
explicit order clause, ActiveRecord sorts records by primary key. This
can result in unpredictable and surprising behaviour when the primary
key is not an auto-incrementing integer, for example when it's a UUID.
This change makes it possible to override the column used for implicit
ordering such that +first+ and +last+ will return more predictable
results. For Example:

  class Project < ActiveRecord::Base
    self.implicit_order_column = "created_at"
  end

1 files changed, 10 insertions, 0 deletions

diff --git a/activerecord/test/cases/finder_test.rb b/activerecord/test/cases/finder_test.rb
index 52fd9291b2..21e84d850b 100644
--- a/activerecord/test/cases/finder_test.rb
+++ b/activerecord/test/cases/finder_test.rb
@@ -741,6 +741,16 @@ class FinderTest < ActiveRecord::TestCase
     assert_equal expected, clients.limit(5).first(2)
   end
 
+  def test_implicit_order_column_is_configurable
+    old_implicit_order_column = Topic.implicit_order_column
+    Topic.implicit_order_column = "title"
+
+    assert_equal topics(:fifth), Topic.first
+    assert_equal topics(:third), Topic.last
+  ensure
+    Topic.implicit_order_column = old_implicit_order_column
+  end
+
   def test_take_and_first_and_last_with_integer_should_return_an_array
     assert_kind_of Array, Topic.take(5)
     assert_kind_of Array, Topic.first(5)