def schema_keys
      schema_file = File.join(Rails.root, 'solr_conf', 'schema.xml')
      file = File.open(schema_file)
      doc = Nokogiri::XML(file)
      file.close
      doc.xpath('//fields/field').map { |f| f.attr('name') }
    end
def schema_keys
      schema_file = File.join(Rails.root, 'solr_conf', 'schema.xml')
      file = File.open(schema_file)
      doc = Nokogiri::XML(file)
      file.close
      doc.xpath('//fields/field').map { |f| f.attr('name') }
    end
Get field names from Solr schema in host application.
 Will raise exception if file not found.
 @return [Array]