plugins/solr/views/search/_search_form_extra_fields.html.erb
<% params_uri = CGI::unescape(request.request_uri) %>
<% if params_uri.index('?') %>
<% params_uri[(params_uri.index('?')+1)..-1].to_s.split("&").each do |part| %>
<% if part.start_with? "facet" %>
<% name_value = part.split("=") %>
<%= hidden_field_tag name_value[0], name_value[1] %>
<% end %>
<% end %>
<% end %>