class AModel extends RelationModel
{
    protected $_link = array(
        'B'=>array(
        'class_name' => 'B',
        'mapping_type' => self::BELONGS_TO,
        'mapping_name' => 'b',
        'foreign_key' => 'bid',//關聯id
    ),
    //c表要間接關聯b表
    'C' => array(
        'class_name' => 'C',
        'mapping_type' => self::MANY_TO_MANY,
        'mapping_name' => 'c',
        'mapping_key' => 'bid', // a.bid, a表要關聯b表的字段
        'foreign_key' => 'id', // c.id 
        'relation_foreign_key' => 'cid', // b.cid b表要關聯c表的字段
        'relation_table' => '__B__', //b表，雙下劃線加大寫，會自動轉換統一格式的表，如果多個大寫例如BFModel,寫成__B_F__中間需要用一個下劃線隔開
    ),
);