How to create a better flash error message

Currently, my flash message looks like this: "Attend was not created.
{:user_id=>["You are already attending this event"]}"

How can i make it return: "Attend was not created. You are already
attending this event"

Controller / def create:

     if @attend.save
       ...
     else
       format.js { flash[:notice] = "Attend was not created.
#{@attend.errors}" }
     end

The closest I've gotten to was:

format.js { flash[:notice] = "Attend was not created.
#{@attend.errors[:user_id]}" }

which returns: Attend was not created. ["You are already attending
this event"]

Would be good, without the [""]