def validate_post_request(post_field, allowed_fields):
    """
    Validators returning None if everything is OK - otherwise suitable HTTP
    status and message is set.
    """
    error_msg = None
    dict_diff = list(set(post_field) - set(allowed_fields))
    if dict_diff:
        error_msg = 'There are fields not allowed in your request.'
    for field in allowed_fields:
        if field not in post_field.keys():
            error_msg = "There is no '{}' field in your request.".format(field)
            break
    return error_msg
