学习JPA又一个错误！
错误名：cannot simultaneously fetch multiple bags
在实体类中映射了几个集合属性，如：
@OneToMany(mappedBy = "customer",cascade = {CascadeType.ALL},fetch = FetchType.EAGER)
private Collection<User> user;
@OneToMany(mappedBy = "customer",cascade = {CascadeType.ALL},fetch = FetchType.EAGER)
private Collection<BookOrder> orders ;
结果Tomcat启动时就报cannot simultaneously fetch multiple bags异常，翻了一下书：在用JPA进行注释时，如果一个实体里要映射多个集合实体时，我们不能把两个集合的的FetchType设置为EAGER,此时只能设置为LAZY,否则会报：cannot simultaneously fetch multiple bags。或者我们也可以借助:@IndexColumn
