docs: function( json, field, params, callback ){
      field = field || 'id';
      var objectIds = [],
        props;
  
      if (params.orderByFields && params.orderByFields !== ''){
        var fld = params.orderByFields.split(' ');
        var order = params.orderByFields;
        if (fld[fld.length-1] == 'DESC'){
          order = '-' + params.orderByFields;
        }
        json.features = this.orderBy(json.features, params.orderByFields, order);
      }
  
      json.features.forEach( function(f){
        props = f.attributes || f.properties;
        objectIds.push( props[ field ] );
      });
  
      callback( null, {
        objectIdField: field,
        objectIds: objectIds
      });
    }