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Subject: RE: Question about @OneToMany
Date: Thu, 8 May 2008 21:54:27 +0200
Message-ID: <12D996978341E849B21C51CC97F4771DB7E17F@mail3.oams.com>
In-Reply-To: <17134828.post@talk.nabble.com>
Thread-Topic: Question about @OneToMany
Thread-Index: AcixQvaIobPQDT4MThCoOfBcWFhYdAAANU+g
References: <480E31E6.5040400@cioppino.net> <480F7AD0.5010708@cs.cmu.edu>
 <17124365.post@talk.nabble.com> <48230AE9.5030505@cs.cmu.edu>
 <17134828.post@talk.nabble.com>
From: "Michael Vorburger" <mvorburger@odyssey-group.com>
To: <users@openjpa.apache.org>

Hello,

Similar=20situation=20here,=20and=20I=20wonder=20about=20people's=20though=
ts=20&=20pros/cons
regarding=20this:

>=20Your=20solution=20(anotate=20"ModelObject=20parent;"=20with=20@ManyToO=
ne=20instead
of=20@Transient=20and=20add=20mappedBy=3D"parent"=20to=20@OneToMany)=20wor=
ks,=20also=20for
my=20application,=20but=20turns=20the=20uni-directional=20relation=20into=20=
a
bi-directional=20relation.=20=20And=20this=20bi-directional=20relation=20i=
s=20more
dificult=20to=20manage=20and=20not=20needed=20in=20our=20Java=20applicatio=
n.

Why=20is=20this=20"bi-directional=20relation=20is=20more=20dificult=20to=20=
manage=20and=20not
needed=20in=20our=20Java=20application"=20?=20=20If=20you=20let=20OpenJPA=20=
deal=20with=20this=20for
you=20with=20the=20Managed=20Inverses
(http://openjpa.apache.org/docs/latest/manual/ref_guide_inverses.html),
what's=20the=20problem=20with=20an=20additional=20field?=20=20Hide=20it=20=
even=20-=20if=20for=20some
reason=20you=20don't=20want=20to=20expose=20the=20bi-directionality=20to=20=
the=20users=20of
the=20object=20model=20for=20keep=20that=20'@ManyToOne=20ModelObject=20par=
ent'=20as=20a
private=20with=20no=20setters=20and=20getters=20-=20just=20to=20get=20the=20=
desired=20mapping!

What's=20the=20issue=20with=20this=20approach?=20=20(Portability=20and=20n=
ot=20wanting=20to
use=20Managed=20Inverses=20aside;=20but=20I=20don't=20know=20if=20that's=20=
strong=20enough,
you're=20using=20OpenJPA=20specific=20configuration=20elsewhere=20already,=
=20and
that's=20really=20what=20this=20is=20-=20a=20runtime=20configuration=20spe=
cific=20to=20one=20JPA
implementation.=20=20Other=20implementations=20may=20have=20a=20similar=20=
feature?=20=20If
not,=20worst=20case,=20when=20switching,=20manually=20code=20the=20logic=20=
[with=20loop
detection,=20if=20needed]).

Regards,
Michael


-----Original=20Message-----
From:=20Uden=20[mailto:udenvh@gmail.com]=20
Sent:=20jeudi,=208.=20mai=202008=2021:37
To:=20users@openjpa.apache.org
Subject:=20Re:=20Question=20about=20@OneToMany


Hi=20Andy,=20thanks=20for=20the=20quick=20response.

In=20my=20application=20i=20had=20three=20tables=20generated=20(a=20master=
,=20a=20detail=20and=20a
join=20table)=20for=20what=20i=20believed=20could=20be=20modelled=20by=20j=
ust=20the=20master=20and
a=20detail=20table=20(at=20least=20in=20traditional=20database=20modeling)=
.=20So=20when=20i
was=20looking=20for=20a=20solution=20to=20simplify=20my=20datamodel=20in=20=
the=20database,=20i
found=20this=20posting=20which=20looked=20similar=20to=20my=20problem=20an=
d=20added=20my
question.
=20
I=20need=20to=20map=20the=20following=20Java=20classes=20(note=20i=20reuse=
=20Marco's=20code
example):

@MappedSuperclass
@Inheritance(strategy=3DInheritanceType.TABLE_PER_CLASS)
class=20ModelObject=20{

=20=20=20=20=20=20=20=20@Id
=20=20=20=20=20=20=20=20private=20int=20id

=20=20=20=20=20=20=20=20@Transient
=20=20=20=20=20=20=20=20ModelObject=20parent;

=20=20=20=20=20=20=20=20@OneToMany(cascade=3D{CascadeType.ALL},=20fetch=3D=
FetchType.EAGER)
=20=20=20=20=20=20=20=20List<ModelObject>=20children;

=20=20=20=20=20=20=20=20.....
}

@Entity
class=20Company=20extends=20ModelObject=20{
}

@Entity
class=20Employer=20extends=20ModelObject=20{
}

to=20the=20folowing=20database=20tables:

COMPANY=20
(ID=20=20=20=20=20(PK)
,=20...
)

and=20EMPLOYEE
(ID=20=20=20=20=20(PK)
,COMPANY_ID=20=20=20=20(FK=20to=20COMPANY.ID)
,=20...
)

In=20Marco's=20original=20question=20he=20indicates=20that=20in=20his=20ca=
se=20"openjpa
create=204=20tables=20(Company,=20Company_children,=20Emplpyer=20and
Employer_children).".=20
Your=20solution=20(anotate=20"ModelObject=20parent;"=20with=20@ManyToOne=20=
instead=20of
@Transient=20and=20add=20mappedBy=3D"parent"=20to=20@OneToMany)=20works,=20=
also=20for=20my
application,=20but=20turns=20the=20uni-directional=20relation=20into=20a
bi-directional=20relation.=20
And=20this=20bi-directional=20relation=20is=20more=20dificult=20to=20manag=
e=20and=20not
needed=20in=20our=20Java=20application.=20It=20does=20however=20results=20=
in=20a=20simpler
datamodel.=20

Your=20second=20solution=20changes=20the=20direction=20(or=20ownership)=20=
of=20the
relation=20in=20the=20Java=20model.

So=20it=20seems=20we=20have=20to=20trade=20a=20simplified=20database=20dat=
amodel=20versus=20a
simplified=20Java=20datamodel.

Or=20is=20there=20an=20other=20solution?

thanks,
Uden


hazen=20wrote:
>=20
>=20Hi=20Uden,
>=20
>=20If=20you'd=20rather=20keep=20the=20relation=20unidirectional=20just=20=
remove=20the=20
>=20`children`=20field--=20the=20`parent`=20field=20alone=20is=20enough=20=
to=20encode=20the=20
>=20relation=20without=20recourse=20to=20a=20separate=20join=20table.=20Ei=
ther=20way,=20I=20
>=20don't=20see=20how=20the=20quoted=20section=20below=20results=20in=20a=20=
join=20table.=20Both=20
>=20fields=20rely=20solely=20on=20a=20"parent_id"=20column=20within=20the=20=
"ModelObject"=20
>=20table,=20not=20on=20some=20other=20join=20table.=20Am=20I=20missing=20=
something?
>=20
>=20Andy
>=20
>=20Uden=20wrote:
>>=20The=20solution=20quoted=20below=20but=20has=20a=20consequence=20for=20=
the=20Java=20class
model:
>>=20the
>>=20OneToMany=20relationship=20becomes=20bi-directional=20instead=20of
uni-directional.=20
>>=20
>>=20What=20is=20the=20reason=20for=20creating=20the=20join-table?=20
>>=20
>>=20I=20thought=20(based=20on=20my=20database=20experience)=20that=20a=20=
join-table=20is=20only

>>=20required=20for=20ManyToMany=20relationships.
>>=20If=20you=20look=20at=20the=20data=20in=20the=20join-table=20of=20a=20=
uni-directional=20
>>=20relation=20(no=20mappedBy=20attribute),=20the=20relation=20between=20=
the=20join-table

>>=20and=20master=20table=20is=20always=20OneToOne,=20so=20this=20relation=
=20could=20be=20
>>=20handled=20by=20a=20FK-field=20in=20the=20detail-table.
>>=20
>>=20thanks=20for=20your=20explanation,
>>=20Uden
>>=20
>>=20
>>=20Andy=20Schlaikjer-2=20wrote:
>>>=20Marco=20Schwarz=20wrote:
>>>>=20How=20can=20I=20make=20only=202=20tables?
>>>=20Here's=20my=20guess:
>>>
>>>=20First,=20use=20the=20"mappedBy"=20property=20of=20the=20@OneToMany=20=
annotation,=20
>>>=20like
>>>=20this:
>>>
>>>=20@Entity
>>>=20class=20ModelObject=20{
>>>=20=20=20=20...
>>>
>>>=20=20=20=20@ManyToOne
>>>=20=20=20=20ModelObject=20parent;
>>>
>>>=20=20=20=20@OneToMany(mappedBy=3D"parent")
>>>=20=20=20=20List<ModelObject>=20children;
>>>
>>>=20=20=20=20...
>>>=20}
>>>
>>>=20This=20way,=20an=20extra=20join=20table=20won't=20be=20necessary=20t=
o=20encode=20the=20
>>>=20parent=20child=20relationship.=20Only=20the=20"parent_id"=20column=20=
in=20the
"ModelObject"
>>>=20table=20will=20be=20used=20to=20encode=20the=20relationship.
>>>
>>=20
>=20
>=20

--
View=20this=20message=20in=20context:
http://www.nabble.com/Question-about-%40OneToMany-tp16840368p17134828.ht
ml
Sent=20from=20the=20OpenJPA=20Users=20mailing=20list=20archive=20at=20Nabb=
le.com.


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