Putting "RE:" in the subject line of Private Messages

Scott Holland wrote:

My app has a private messaging system and I need to be able to put “Re:”
in the subject line when a user replies to a message.

[...]

<input type="text" value="Re: <%= @message.subject %>">

[...]

My problem is how do I stop the “Re:” building up when I save it to the
database.

In the above case it could be done with something like:

<%= 'Re: ' unless @message.subject.starts_with?('Re:') %>

But it strikes me as something that's better placed in the model, not
the UI.