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再问一个OR Mapping的问题

再问一个OR Mapping的问题# Java - 爪哇娇娃

A*o2009-03-23 07:03

1 楼

一个Customer Entity，有两个Set，分别叫privateEmails和publicEmails，
里面都是Email。难道就要映射到两个Email表上去？还有没有更简洁的方案？
谢谢！

g*g2009-03-23 07:03

2 楼

映射到同一个email表上有啥问题？

【在 A**o 的大作中提到】

: 一个Customer Entity，有两个Set，分别叫privateEmails和publicEmails，
: 里面都是Email。难道就要映射到两个Email表上去？还有没有更简洁的方案？
: 谢谢！

A*o2009-03-23 07:03

3 楼

same forgein key on email table,
how can you tell it's private or public?

【在 g*****g 的大作中提到】

: 映射到同一个email表上有啥问题？

b*y2009-03-23 07:03

4 楼

不会吧，难道foreign key跟哪个field都不能specify?
逼你去duplicate email table?
老夫觉得难以相信啊

【在 A**o 的大作中提到】

: same forgein key on email table,
: how can you tell it's private or public?

t*e2009-03-23 07:03

5 楼

table per hierarchy.
Define an abstract entity - Email.
@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(
name="Inheritance",
discriminatorType=DiscriminatorType.STRING
)
public astract class Email{
}
followed by,
@Entity
@DiscriminatorValue("Public")
public class PublicEmail{
}
@Entity
@DiscriminatorValue("Private")
public class PrivateEmail{
}

A*o2009-03-23 07:03

6 楼

Thank you! How do you map the one to many to Customer, then?
Does it mean that I map one to PublicEmail and another to PrivateEmail?
That looks cool.

【在 t*******e 的大作中提到】

: table per hierarchy.
: Define an abstract entity - Email.
: @Entity
: @Inheritance(strategy = InheritanceType.SINGLE_TABLE)
: @DiscriminatorColumn(
: name="Inheritance",
: discriminatorType=DiscriminatorType.STRING
: )
: public astract class Email{
: }

m*t2009-03-23 07:03

7 楼

Add a TYPE field to the email table. And use the 'where' attribute on the
set mapping to set the filtering condition.

【在 A**o 的大作中提到】

: 一个Customer Entity，有两个Set，分别叫privateEmails和publicEmails，
: 里面都是Email。难道就要映射到两个Email表上去？还有没有更简洁的方案？
: 谢谢！

A*o2009-03-23 07:03

8 楼

Any sample (pseudo) code? or the annotation i should look for?
i like totempole's solution for now.

【在 m******t 的大作中提到】

:
: Add a TYPE field to the email table. And use the 'where' attribute on the
: set mapping to set the filtering condition.

m*t2009-03-23 07:03

9 楼

Not sure about the equivalent annotation. I don't use those for
OR mapping.
That's certainly another way of doing it. I like the where approach
because I feel it's a bit of an overkill to introduce
different classes only for filtering purposes - kind of like bending
the OO design backwards to fit into the ORM tool. But it's really
a personal taste thing I guess.

【在 A**o 的大作中提到】

: Any sample (pseudo) code? or the annotation i should look for?
: i like totempole's solution for now.

t*e2009-03-23 07:03

10 楼

@OneToMany如果能加where clause就最好了，没看到这样的例子。 @
DiscriminatorValue里面是可以加where clause的.

g*g2009-03-23 07:03

11 楼

+1, not worth the trouble when a simple wrapper in DAO can do it.

【在 m******t 的大作中提到】

:
: Not sure about the equivalent annotation. I don't use those for
: OR mapping.
: That's certainly another way of doing it. I like the where approach
: because I feel it's a bit of an overkill to introduce
: different classes only for filtering purposes - kind of like bending
: the OO design backwards to fit into the ORM tool. But it's really
: a personal taste thing I guess.

F*n2009-03-23 07:03

12 楼

Actually they are same. When you declare "one table per hierarchy" it will
create a special "descriminant" column in the table to tell which subclass
the data record belongs to. This is equivalent to your explicit "type"
column -- it is just managed transparently to developers.

【在 m******t 的大作中提到】

A*o2009-03-23 07:03

13 楼

yeah. that transparency is what i like. less work for me. :)

【在 F****n 的大作中提到】

: Actually they are same. When you declare "one table per hierarchy" it will
: create a special "descriminant" column in the table to tell which subclass
: the data record belongs to. This is equivalent to your explicit "type"
: column -- it is just managed transparently to developers.

A*o2009-03-23 07:03

14 楼

OK, after reading Hibernate doc, I figured the Unidirectional with join
table strategy suits my case best. Because the email is a common object and
shared across the objects.
The follow is quote from hibernate:
http://www.hibernate.org/hib_docs/annotations/reference/en/html/entity.html
A unidirectional one to many with join table is much preferred. This
association is described through an @JoinTable.
@Entity
public class Trainer {
@OneToMany
@JoinTable(
name="TrainedMonkeys",

t*e2009-03-23 07:03

15 楼

This is a good choice.
In Hibernate, you may instead use collections of components because theoretically
email/address are embeddable objects (2nd class entities),
@CollectionOfElements(
targetElement = java.lang.String.class
)
@JoinTable(
name = "UserEmails",
[email protected](name="UserId")
)
@Column(name="Email", nullable=false)
@OrderBy(clause="Email ASC")
private Set emails;
Without a join table, the reverse pointer/foreign k

【在 A**o 的大作中提到】

: OK, after reading Hibernate doc, I figured the Unidirectional with join
: table strategy suits my case best. Because the email is a common object and
: shared across the objects.
: The follow is quote from hibernate:
: http://www.hibernate.org/hib_docs/annotations/reference/en/html/entity.html
: A unidirectional one to many with join table is much preferred. This
: association is described through an @JoinTable.
: @Entity
: public class Trainer {
: @OneToMany

A*o2009-03-23 07:03

16 楼

i'm stuck on jboss ejb3 for the moment, no hibernate extenstions for now.
but since it's based on hibernate, i just read their doc for more info.
again, thanks for your help, all of you who replied.

【在 t*******e 的大作中提到】

: This is a good choice.
: In Hibernate, you may instead use collections of components because theoretically
: email/address are embeddable objects (2nd class entities),
: @CollectionOfElements(
: targetElement = java.lang.String.class
: )
: @JoinTable(
: name = "UserEmails",
: [email protected](name="UserId")
: )