association load target shouldn't replace records

from db if it is already loaded by nested attributes assignment

[#5053 state:resolved]
Signed-off-by: NJosé Valim <jose.valim@gmail.com>

activerecord/lib/active_record/associations/association_collection.rb

@@ -396,11 +396,12 @@ def load_target
                 if @target.is_a?(Array) && @target.any?
                   @target = find_target.map do |f|
                     i = @target.index(f)
-                    t = @target.delete_at(i) if i
-                    if t && t.changed?
+                    if i
+                      t = @target.delete_at(i)
+                      keys = ["id"] + t.changes.keys + (f.attribute_names - t.attribute_names)
+                      t.attributes = f.attributes.except(*keys)
                       t
                     else
-                      f.mark_for_destruction if t && t.marked_for_destruction?
                       f
                     end
                   end + @target

activerecord/test/cases/nested_attributes_test.rb

@@ -862,7 +862,19 @@ def setup
     assert_equal 1, @ship.parts.proxy_target.size
     assert_equal 'Deck', @ship.parts[0].name
   end
-  
+
+  test "if association is not loaded and child doesn't change and I am saving a grandchild then in memory record should be used" do
+    @ship.parts_attributes=[{:id => @part.id,:trinkets_attributes =>[{:id => @trinket.id, :name => 'Ruby'}]}]
+    assert_equal 1, @ship.parts.proxy_target.size
+    assert_equal 'Mast', @ship.parts[0].name
+    assert_no_difference("@ship.parts[0].trinkets.proxy_target.size") do
+      @ship.parts[0].trinkets.proxy_target.size
+    end
+    assert_equal 'Ruby', @ship.parts[0].trinkets[0].name
+    @ship.save
+    assert_equal 'Ruby', @ship.parts[0].trinkets[0].name
+  end
+
   test "when grandchild changed in memory, saving parent should save grandchild" do
     @trinket.name = "changed"
     @ship.save