Hey Chris,
Thanks for your tip, but we (and indeed I forgot to mention this) already do
the query on m:n
objects like this (except we use 'hard' table names and PK field names).
What I was wondering
was if there was an easier way to do it than like this :)
For example something like this would be extremely cool to have in OJB:
Criteria c = new Criteria();
c.addIn("bees[]", B);
Query q = new QueryByCriteria(B.class, c, false);
broker.getCollectionByQuery(q);
or even
Criteria c = new Criteria();
c.addEqualTo("bees[0].name", B.getName());
Query q = new QueryByCriteria(B.class, c, false);
broker.getCollectionByQuery(q);
Nevertheless, thanks for your input! :)
----- Original Message -----
From: "Chris Lewington" <[EMAIL PROTECTED]>
To: "OJB Users List" <[EMAIL PROTECTED]>
Sent: Thursday, October 03, 2002 2:28 PM
Subject: Re: Query on collections
> Hi Rudi,
>
> I had to address a similar problem as part of implementing a "query by
object"
> mechanism (tricky!) to search on "partially complete" objects rather than
using
> straight queries themselves.. The basic idea I had was as follows:
>
> - build an empty A and set your concrete B into its collection (use this
as a
> basis for what follows);
>
> - use the CollectionDescriptor of class A to get the FK column names of
the
> indirection table to both the item class (B) and the "this" class (A), and
also
> the name of the indirection table itself;
>
> - then, from your concrete B, use its ClassDescriptor to get the PK field
info
> and hence from that a PersistentField from which you can obtain the actual
PK
> value of your B object;
>
> - then build a Criteria object with the following:
> 1) an "addEqualToColumn" which matches the PK field for A to its
> corresponding column name in the indirection table, plus
> 2) an "addEqualTo" which matches the actual PK value you have for B with
its
> corresponding column name in the indirection table;
>
> - then finally create a "QueryByMtoNCriteria" with the class of A, the
> indirection table name and the newly built Criteria object.
>
> From there you can do a getCollectionByQuery with your newly built Query
object
> above. I'm sure there are easier ways but that was the easiest I could
come up
> with for now and it works OK for me.
>
> Hope that helps,
>
> Cheers,
>
>
> Chris
>
> Rudi Alberda wrote:
>
> > Hi all,
> >
> > I have a question regarding queries on m:n relations. Suppose I have two
> > classes which are associated in OJB through a m:n relation, objects of
type
> > 'A' and 'B'. 'A' has a property called 'bees' returning all objects of
type
> > 'B', 'B' has a property called 'as' which returns all 'A' objects.
> > This all works nicely in OJB, storing, updating, deleting. But now I
want to
> > query. I have an object of type 'B' and I want to know which objects of
type
> > 'A' are associated to it. In other words, I'd like a query which results
in
> > a collection of 'A' objects for which 'B' is in the collection 'A.bees'.
> > How would this work?
> > I cannot simply go and create a query like:
> >
> > Criteria c = new Criteria();
> > c.addEqualTo("bees", B);
> > Query q = new QueryByCriteria(B.class, c, false);
> > broker.getCollectionByQuery(q);
> >
> > because bees is a collection. Will Criteria.addIn work? And if this
doesn't,
> > what will?
> > I have looked at QueryByMtoNCriteria but the documentation of this was
> > rather unclear.
> > Also, I might be able to use Criteria.addExists, but I need to refer to
a
> > field from the main query in the subquery, otherwise my subQuery isn't
of
> > much use.
> > Help!
> >
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