function _getIdProps(source){
            var result = [];

            if(source instanceof Array){
                source.forEach(function(obj){
                    if(obj && obj.id){
                        result.push(obj.id);
                    }
                });
            } else if(source && source.id){
                result.push(source.id);
            }

            return (result.length > 0) ? result : undefined;
        }
Returns "id" properties of given object.
If {@code source} is array, extract "id" property of each object in the array.

@param source
@returns {Array}
@private