I think I'm a bit confused about how this would be mapped in JPA
annotations.
If I understand correctly Jeff's data model is something like:
_COMPUTER_
ID PK
_PORT_
TYPE PK
COMPUTER_ID PK FK
This means that Port has a composite primary key.
@Embeddable
class PortPK {
private String type;
private Computer computer;
}
@Entity
class Port {
@Id
PortPK portId;
}
@Entity
class Computer {
@Id
private Long id;
@OneToMany
@JoinColumn(name="COMPUTER_ID")
private Set<Port> ports;
}
This would represent a uni-directional OneToMany between Computer and Port.
i.e. A computer knows about it's ports but a port doesn't know about it's
computer (other than the fact that the port pk includes the computer - this
is what is confusing me a bit).
I don't think TopLink essentials supports the use of @JoinColumn on a
uni-directional OneToMany relationship. At least, I think I raised a bug
about this a while ago, but do not know if that has been addressed.
If this was to be setup as a bi-directional relationship what would it look
like?
1. Just drop the @JoinColumn?
2. or Add a @ManyToOne annotation to the PortPK Computer field?
3. or something else?
Thanks
Martin
-----Original Message-----
From: Marina.Vatkina_at_Sun.COM [mailto:Marina.Vatkina_at_Sun.COM]
Sent: March 14, 2007 3:54 PM
To: persistence_at_glassfish.dev.java.net
Subject: Re: question about object relationships
If you start with Java classes, and would like for the provider to generate
the
tables for you, you'll need to specify the desired mapping, but if you map
to an
existing schema, part of the PK in the Port (e.g. computerId) will be a FK
to
the PK in the Computer, and another field (e.g. type) will be just a PK
field.
In this case a combination of 2 fields in the Post will guarantee that
ComputerA
and ComputerB can have ports of the same type.
HTH,
-marina
jeff wrote On 03/14/07 14:38,:
> i still don't understand then. if there's a unidirectional relationship
> from one Computer to many Ports, then Port is only identified by it's
> type. If that's the case, then it's impossible for ComputerA and
> ComputerB to have ports of the same type.
>
> a Port's ID must somehow incorporate the Computer that it belongs to,
> and the only way i can see to do that is to have a bi-directional
> relationship. or, to manually (or automatically) assign each port a
> unique ID.
>
> is that correct?
>
> thanks.
>
> */Marina Vatkina <Marina.Vatkina_at_Sun.COM>/* wrote:
>
> It should work either way.
>
> -marina
>
> jeff wrote On 03/14/07 14:14,:
> > thanks marina, i did see that.
> >
> > then i think my question becomes: is it necessary to have a
> > bidirectional relationship to affect this behavior?
> >
> > thanks.
> >
> > */Marina Vatkina /* wrote:
> >
> > Look at the orderapp in the JavaEE tutorial - it has orders and
> > lineitems with
> > overlapping PK/FK mapping. This is most probably what you need.
> >
> > regards,
> > -marina
> >
> > jeff wrote On 03/14/07 11:25,:
> > > in a one to many (or 1-1) relationship, i want the many's ID to
> > > incorporate it's one's ID.
> > >
> > > for example, if a have a Computer that has Ports. each port has a
> > type,
> > > say "USB", "serial", etc. furthermore, say i can only have one
> of each
> > > type of port in a particular Computer.
> > >
> > > many Computers can have a Ports of the same type. how do i
represent
> > > this? what is the ID for Port? the type is enough to ID it
> within the
> > > Computer, but i don't know how to represent this in JP. i think
> i want
> > > the Port's ID to incorporate the Computer's ID.
> > >
> > > in the database, i think this would mean that the table for Port
> has a
> > > foreign key pointing back to the the ID column of the table for
> > Computer.
> > >
> > > i hope this makes sense.
> > > thanks!
> > >
> > >
> > >
> >
>
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