app/views/admin/matches/_editable_team.html.erb
<% team_id = "#{team}_id".to_sym %>
<%= form.label team_id %>
<%= form.select team_id, match.teams_select_options(team_id), {include_blank: true}, {class: ('error' if match.errors.include?(team_id))} %>
<% if match.errors.include?(team_id) %>
<small class="error"><%= match.errors.get(team_id).to_sentence %></small>
<% end %>