<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def init(req, model):
 """ Determine the sorting preference by query parameter Return an array of Sortable objects. """


    rels = model.relationships
    fields = model.all_fields

    params = req.get_param_as_list('sort') or [goldman.config.SORT]
    params = [Sortable(param.lower()) for param in params]

    for param in params:
        _validate_no_rels(param, rels)
        _validate_field(param, fields)

    return params