Make CollectionAssociation first/last with integer fetch with query

When first or last is called with an integer on an unloaded association,
the entire collection is loaded. This differs surprisingly from the
behavior of Relation#first/last, which translate the call into a limit
query. For large collections this can make a big difference in
performance.

Change CollectionAssociation#fetch_first_or_last_using_find? to make
this kind of call delegate to Relation.

activerecord/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   `CollectionAssociation#first`/`#last` (e.g. `has_many`) use a `LIMIT`ed
+    query to fetch results rather than loading the entire collection.
+
+    *Lann Martin*
+
 *   Re-use `order` argument pre-processing for `reorder`.
 
     *Paul Nikitochkin*

activerecord/lib/active_record/associations/collection_association.rb

@@ -528,15 +528,13 @@ module ActiveRecord
         #   * target already loaded
         #   * owner is new record
         #   * target contains new or changed record(s)
-        #   * the first arg is an integer (which indicates the number of records to be returned)
         def fetch_first_or_last_using_find?(args)
           if args.first.is_a?(Hash)
             true
           else
             !(loaded? ||
               owner.new_record? ||
-              target.any? { |record| record.new_record? || record.changed? } ||
-              args.first.kind_of?(Integer))
+              target.any? { |record| record.new_record? || record.changed? })
           end
         end
 

activerecord/test/cases/associations/has_many_associations_test.rb

@@ -1414,15 +1414,17 @@ class HasManyAssociationsTest < ActiveRecord::TestCase
     end
   end
 
-  def test_calling_first_or_last_with_integer_on_association_should_load_association
+  def test_calling_first_or_last_with_integer_on_association_should_not_load_association
     firm = companies(:first_firm)
+    firm.clients.create(:name => 'Foo')
+    assert !firm.clients.loaded?
 
-    assert_queries 1 do
+    assert_queries 2 do
       firm.clients.first(2)
       firm.clients.last(2)
     end
 
-    assert firm.clients.loaded?
+    assert !firm.clients.loaded?
   end
 
   def test_calling_many_should_count_instead_of_loading_association