code: function(fieldName) {
                  return _.has(this.filterableFields, fieldName) &&
                      !_.isUndefined(this.filterableFields[fieldName].displayName);
              }
docs: Returns whether this collection has defined a given
  filterable field.
  
  @param fieldName {string} querystring parameter name for the
  filterable field
  @return {boolean} Returns true if this collection has the specified field.