Rails session Couldn't find User with username = signout
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I'm working through the Michael Hartl Tutorial and am having trouble getting sign-outs to work.

Instead of ID'ing by email, I'm working by username.  

Signing up and singing in both work fine, but my sign out causes an error:

    ActiveRecord::RecordNotFound in UsersController#show

    Couldn't find User with username = signout

    app/controllers/users_controller.rb:9:in `show'

Why is it looking in the users_controller, in the show method?  Why is it calling that?  It should be calling sessions controller's delete method, right?

The offending line 9 is---

      8  def show
      9     @user = User.find_by_username!(params[:username])
      10 end

I don't understand where the parameters are getting passed in (where and to what) that cause it to think "signout" == the username, rather than finding by username and routing to session#destroy.  How can I diagnose this?  What can I type in the rails console to find out what is currently contained in params?