mysql 1215报错解决求教！！！

先用的联合主键
alter table project add constraint pk_project primary key (p_id,PID);

然后在新建表中就无法使用其做外键了T^T
我写的是
create table relation（projectp_id int(20),projectPID int(20), foreign key(projectp_id,projectPID) references project(p_id,PID)）;

然后就会报错。。。cannot add foreign key constraint
why??????????????
求教！！！！！！！！！！！

http://blog.csdn.net/sinat_31726559/article/details/51712069