def IF_NULL(expression, _else):
    """
    Evaluates an expression and returns the value of the expression if the expression evaluates to a non-null value.
    See https://docs.mongodb.com/manual/reference/operator/aggregation/ifNull/
    for more details
    :param expression: expression that will be evaluated
    :param _else: value or expression if 'expression' is null
    :return:  Aggregation operator
    """
    return {'$ifNull': [expression, _else]}
