Create associations between User and Listing tables. (Make sure to log in.) Display user's :name on listing in index.html.erb. WILL GET ERROR from old records that don't have user_id association, which cannot display :name via the view.
Temporarily remove <h3><%= listing.name %></h3> to delete old records, which are causing the error b/c there is no user assiciated with listings, so you can't display their :name in the view.
Deleted wrong line. Revert last deletion.  Delete correct line instead.
Deleted all records. Code now correct for new records.