`has_one` and `belongs_to` accessors don't add ORDER BY to the queries anymore.

Since Rails 4.0, we add an ORDER BY in the `first` method to ensure consistent
results among different database engines. But for singular associations this
behavior is not needed since we will have one record to return. As this
ORDER BY option can lead some performance issues we are removing it for singular
associations accessors.

Fixes #12623.

activerecord/CHANGELOG.md

+*   `has_one` and `belongs_to` accessors don't add ORDER BY to the queries anymore.
+
+    Since Rails 4.0, we add an ORDER BY in the `first` method to ensure consistent results
+    among different database engines. But for singular associations this behavior is not needed
+    since we will have one record to return. As this ORDER BY option can lead some performance
+    issues we are removing it for singular associations accessors.
+
+    Fixes #12623.
+
+    *Rafael Mendonça França*
+
 *   Prepend table name for column names passed to `Relation#select`.
 
     Example:

activerecord/lib/active_record/associations/singular_association.rb

@@ -39,7 +39,7 @@ def create_scope
         end
 
         def find_target
-          if record = scope.first
+          if record = scope.take
             set_inverse_instance record
           end
         end

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -28,6 +28,13 @@ def test_belongs_to
     assert_equal companies(:first_firm).name, firm.name
   end
 
+  def test_belongs_to_does_not_use_order_by
+    ActiveRecord::SQLCounter.clear_log
+    Client.find(3).firm
+  ensure
+    assert ActiveRecord::SQLCounter.log_all.all? { |sql| /order by/i !~ sql }, 'ORDER BY was used in the query'
+  end
+
   def test_belongs_to_with_primary_key
     client = Client.create(:name => "Primary key client", :firm_name => companies(:first_firm).name)
     assert_equal companies(:first_firm).name, client.firm_with_primary_key.name

activerecord/test/cases/associations/has_one_associations_test.rb

@@ -22,6 +22,13 @@ def test_has_one
     assert_equal Account.find(1).credit_limit, companies(:first_firm).account.credit_limit
   end
 
+  def test_has_one_does_not_use_order_by
+    ActiveRecord::SQLCounter.clear_log
+    companies(:first_firm).account
+  ensure
+    assert ActiveRecord::SQLCounter.log_all.all? { |sql| /order by/i !~ sql }, 'ORDER BY was used in the query'
+  end
+
   def test_has_one_cache_nils
     firm = companies(:another_firm)
     assert_queries(1) { assert_nil firm.account }