Call build when extend with nested attributes defined

What?
From now on when `accepts_nested_attributes_for` defined and `extend` option
added the overwritten `build` method being called.

[Alireza Bashiri, Martins Polakovs]

activerecord/lib/active_record/nested_attributes.rb

@@ -501,7 +501,7 @@ module ActiveRecord
 
           if attributes["id"].blank?
             unless reject_new_record?(association_name, attributes)
-              association.build(attributes.except(*UNASSIGNABLE_KEYS))
+              association.reader.build(attributes.except(*UNASSIGNABLE_KEYS))
             end
           elsif existing_record = existing_records.detect { |record| record.id.to_s == attributes["id"].to_s }
             unless call_reject_if(association_name, attributes)

activerecord/test/cases/nested_attributes_test.rb

@@ -1094,3 +1094,15 @@ class TestHasManyAutosaveAssociationWhichItselfHasAutosaveAssociations < ActiveR
     assert_equal ["Ship name can't be blank"], part.errors.full_messages
   end
 end
+
+class TestNestedAttributesWithExtend < ActiveRecord::TestCase
+  setup do
+    Pirate.accepts_nested_attributes_for :treasures
+  end
+
+  def test_extend_affects_nested_attributes
+    pirate = Pirate.create!(catchphrase: "Don' botharrr talkin' like one, savvy?")
+    pirate.treasures_attributes = [{ id: nil }]
+    assert_equal "from extension", pirate.treasures[0].name
+  end
+end

activerecord/test/models/pirate.rb

@@ -17,7 +17,13 @@ class Pirate < ActiveRecord::Base
     after_remove: proc { |p, pa| p.ship_log << "after_removing_proc_parrot_#{pa.id}" }
   has_and_belongs_to_many :autosaved_parrots, class_name: "Parrot", autosave: true
 
-  has_many :treasures, as: :looter
+  module PostTreasuresExtension
+    def build(attributes = {})
+      super({ name: "from extension" }.merge(attributes))
+    end
+  end
+
+  has_many :treasures, as: :looter, extend: PostTreasuresExtension
   has_many :treasure_estimates, through: :treasures, source: :price_estimates
 
   has_one :ship