code: function(record, foreign_key) {
  
  		if (foreign_key in this._collections_by_foreign_id) {
  
  			this._collections_by_foreign_id[foreign_key].push(record);
  
  		} else {
  
  			this._collections_by_foreign_id[foreign_key] = [record];
  
  		}
  
  	}
docs: Add record to local collection of records, grouped by this field
  @param {Lava.data.Record} record
  @param {string} foreign_key