连贯操作field用于指定返回的字段
$model->field('id,name')->select();
$model->field(array('id','name'))->select();
生成的SQL语句:`select id,name from table`

$model->field('id,name')->select();
$model->field(array('id','name'))->select();
生成的SQL语句:`select id,name from table`