Fix attaching blobs via nested attributes

Closes #37411.

activestorage/lib/active_storage/attached/changes/create_one.rb

@@ -65,7 +65,7 @@ def find_or_build_blob
             **attachable.reverse_merge(
               record: record,
               service_name: attachment_service_name
-            )
+            ).symbolize_keys
           )
         when String
           ActiveStorage::Blob.find_signed!(attachable, record: record)

activestorage/test/models/attached/one_test.rb

@@ -319,6 +319,13 @@ def upload.open
     assert_equal 2736, @user.avatar.metadata[:height]
   end
 
+  test "creating an attachment as part of an autosave association through nested attributes" do
+    group = Group.create!(users_attributes: [{ name: "John", avatar: { io: StringIO.new("STUFF"), filename: "town.jpg", content_type: "image/jpg" } }])
+    group.save!
+    new_user = User.find_by(name: "John")
+    assert new_user.avatar.attached?
+  end
+
   test "updating an attachment as part of an autosave association" do
     group = Group.create!(users: [@user])
     @user.avatar = fixture_file_upload("racecar.jpg")

activestorage/test/test_helper.rb

@@ -127,6 +127,8 @@ class User < ActiveRecord::Base
 class Group < ActiveRecord::Base
   has_one_attached :avatar
   has_many :users, autosave: true
+
+  accepts_nested_attributes_for :users
 end
 
 require_relative "../../tools/test_common"