diff --git a/activerecord/lib/active_record/associations/association_collection.rb b/activerecord/lib/active_record/associations/association_collection.rb
index 52d2a98..bbd8af7 100644
--- a/activerecord/lib/active_record/associations/association_collection.rb
+++ b/activerecord/lib/active_record/associations/association_collection.rb
@@ -187,7 +187,7 @@ module ActiveRecord
         if @owner.new_record? || (loaded? && !@reflection.options[:uniq])
           @target.size
         elsif !loaded? && !@reflection.options[:uniq] && @target.is_a?(Array)
-          unsaved_records = Array(@target.detect { |r| r.new_record? })
+          unsaved_records = @target.select { |r| r.new_record? }
           unsaved_records.size + count_records
         else
           count_records
diff --git a/activerecord/test/cases/associations/has_many_associations_test.rb b/activerecord/test/cases/associations/has_many_associations_test.rb
index dbfa025..e75bb6a 100644
--- a/activerecord/test/cases/associations/has_many_associations_test.rb
+++ b/activerecord/test/cases/associations/has_many_associations_test.rb
@@ -356,6 +356,13 @@ class HasManyAssociationsTest < ActiveRecord::TestCase
     assert_equal 2, company.clients_of_firm(true).size
   end
 
+  def test_collection_size_after_building
+    company = companies(:first_firm)  # company already has one client
+    company.clients_of_firm.build("name" => "Another Client")
+    company.clients_of_firm.build("name" => "Yet Another Client")
+    assert_equal 3, company.clients_of_firm.size
+  end
+
   def test_build_many
     company = companies(:first_firm)
     new_clients = assert_no_queries { company.clients_of_firm.build([{"name" => "Another Client"}, {"name" => "Another Client II"}]) }