openproject/app/views/customizable/_form.html.erb

<%#-- copyright
OpenProject is a project management system.
Copyright (C) 2012-2018 the OpenProject Foundation (OPF)

This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License version 3.

OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
Copyright (C) 2006-2017 Jean-Philippe Lang
Copyright (C) 2010-2013 the ChiliProject Team

This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License
as published by the Free Software Foundation; either version 2
of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.

See docs/COPYRIGHT.rdoc for more details.

++#%>

<%# locals: form, all_fields, only_required #%>

<% form.object.custom_field_values.each do |value| %>
  <%= form.fields_for_custom_fields :custom_field_values, value do |value_form| %>
    <% required = value.custom_field.is_required? %>
    <div class="form--field <%= required ? '-required' : '' %>">
      <%# display all fields OR only display required fields OR only display optional fields #%>
      <% if all_fields || (only_required && required) || (!only_required && !required) %>
        <%= value_form.custom_field(container_class: '-wide', required: required) %>
      <% end %>
    </div>
  <% end %>
<% end %>