1. When getting error:

ActionView::Template::Error (undefined method `arrange' for #Array:0x007f8adbf76940)

Because arrange method is Ancestry 1.2.5 's class method, I changed

app/views/index.html

<%= nested_messages @messages.arrange(:order => :created_at) %>

into:

<%= nested_messages @messages %>

And changed

app/controller/message_controller.rb

def index
    @messages = Message.all
    ...

end

into:

def index
    @messages = Message.arrange(:order => :created_at)
    ...
end