fastdb中的relation

RELARION-關係，在fastdb內存數據庫中表示在類（表）之間指定是一對一，一對多，還是多對多關係。

例如：class Detail {
  public:
    char const* name;
    char const* material;
    char const* color;
    real4       weight;

    dbArray< dbReference<Contract> > contracts;

    TYPE_DESCRIPTOR((KEY(name, INDEXED|HASHED),
                     KEY(material, HASHED),
                     KEY(color, HASHED),
                     KEY(weight, INDEXED),
                     RELATION(contracts, detail)));
};

class Supplier {
  public:
    char const* company;
    char const* location;
    bool        foreign;

    dbArray< dbReference<Contract> > contracts;

    TYPE_DESCRIPTOR((KEY(company, INDEXED|HASHED),
                     KEY(location, HASHED),
                     FIELD(foreign),
                     RELATION(contracts, supplier)));
};


class Contract {
  public:
    dbDateTime            delivery;
    int4                  quantity;
    db_int8               price;
    dbReference<Detail>   detail;
    dbReference<Supplier> supplier;

    TYPE_DESCRIPTOR((KEY(delivery, HASHED|INDEXED),
                     KEY(quantity, INDEXED),
                     KEY(price, INDEXED),
                     RELATION(detail, contracts),
                     RELATION(supplier, contracts)));
};

Detail和Contract是一對多的關係，Supplier和Contract也是一對多的關係。在每個類中最好的標記符號就是dbArray和dbReference。