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Subject: RE: Joins in torque
To: "Apache Torque Users List" <torque-user@db.apache.org>
Message-ID: 
 <OFFE5ECE37.C9107820-ONC12570FB.005B3863-C12570FB.005D2787@seitenbau.net>
From: Thomas Fischer <fischer@seitenbau.net>
Date: Thu, 19 Jan 2006 17:57:28 +0100
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Hi,

There is no built-in support at the moment to get more than one level o=
f
indrection read in at once.
As far as I can see, you have 3 1/2 possibilities:

1a) read the values one after another. Assuming you have defined foreig=
n
keys (I'm not sure whether they should defined in T1 and T2 or in T12, =
on
this depends whether the doSelectJoinXXX methods are generated in T1Pee=
r
and T2Peer or in T12Peer), do T1Peer.doSelectJoinT12(). For each T1Entr=
y
you get, fetch the corresponding T12 Entry (this is already read from t=
he
database) and then do T12.getT2s() (this will access the database each =
time
you fetch a T2 Entry).

1b) do the same as in a1, but omit the T12.getT2s() step. Instead, get =
all
T2-Ids from all T12 you read in, do T2Peer.doSelect() with a criteria w=
here
all needed T2Ids are selected, and do the linking of the T2 objects to =
the
T12's manually.

2) look at the template code for generating the doSelectJoinXXX methods=
 and
extend them for two levels of indirection.

3) A time ago, someone proposed a class which can read a whole tree of
objects at once. It can be found in scarab (somewhere around TRQS260). =
It
is rather reflection-centric and I do not have an idea whether it works=

with Torque 3.2, but maybe that is what you want.

Personally, I use a modified method 1b). I would read all T1 I want, ga=
ther
the collection of T1 ids to read the associated T12's and link them
manually to the T1's. Then I'd use the T2 ids from there to read the T2=
's,
and link them manually to the T12's. This is rather fast (one select pe=
r
level of indirection, no object is transferred twice) and quite easy to=

implement. You might have to override the initCollXXX() methods for the=

used collections to make them public.

    Thomas


"Stephan Spiegel" <spiegel@szwo.de> schrieb am 19.01.2006 17:17:29:

> Hi, (in case this message is double please excuse me)
>
> I really do not understand what to do, I do not get the expected resu=
lts!
>
> Let=B4s say I have 3 Tables: T1, T2 and T1_2
> T1:
> ID (integer pk)
> value (varchar)
> ...
>
> T2:
> ID (integer pk)
> value (varchar)
> ...
>
> T1_2:
> T1_ID (integer fk->T1)
> T2_ID (integer fk->T2)
>
> As a result, I would like to have a union table showing the values of=
 T1
and
> T2
> in SQL this would be: select T1.value, T2.value from T1, T2, T1_2 whe=
re
> T1.ID =3D T1_2.T1_ID and T2.ID =3D T1_2.T2_ID
>
> if I get more than this (select * ...) would be fine as well.
>
> So, I have my classes but when I try to use them as written in Peers
HowTo
> (doSelectJoin...) or in Criteria HowTo (crit.addJoin(...)) I always g=
et
only
> the values of the T1_2-table
>
> As a first step, I try just to get the first half (result should show=
:
> T1.value, T1_2.T2_ID)
> so it should correspond to:
> doSelectJoinT1(crit) in T1_2Peer
> or:
> crit.addJoin(T1.ID,T1_2.T1_ID,Criteria.LEFT_JOIN)
>
> as a result I just get the T1_2 values. I checked the
mailing-list-archives,
> I can=B4t find a helpful message. Where is my mistake? Could somebody=
 tell
me
> exactly how to add this join?
>
> (I=B4m using torque-3.2, MySQL 5, defaultidMethod=3Dnative)
>
> thanks in advance, Stephan
>
>
>
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