redmine/app/views/imports/_time_entries_mapping.html.erb

<fieldset class="box tabular">
  <legend><%= l(:label_fields_mapping) %></legend>
  <div id="fields-mapping">
    <%= render :partial => 'time_entries_fields_mapping' %>
  </div>
</fieldset>

<%= javascript_tag do %>
$(document).ready(function() {
  $('#fields-mapping').on('change', '#import_mapping_project_id', function(){
    $.ajax({
      url: '<%= import_mapping_path(@import, :format => 'js') %>',
      type: 'post',
      data: $('#import-form').serialize()
    });
  });
});
<% end %>