One of the most interesting uses of an O/R mapper for me is to easily build
hierarchical structures. I have tried it with lazy loading, but there still
remain some problems. Since the tree I have to load is quite large, I don't
want to load a lot at a time, but use lazy loading. The problem below occurs
in both ways, though.
I have a class A with childs of the same class, connected through a "parent"
column in the database.
For this OQL select operation:
SELECT c from A c where c.parent.id = $1
the generated SQL looks like this:
SELECT A.id,
A_f0.id,
A.parent
FROM A A_0,
A A_f0,
A
WHERE
A.id *= A_f0.id
AND
A.parent=A_0.id
AND (A_0.id = ?)
(Using SQL Server; I removed the quotes)
It seems to me that A_0 designates the parent object, A the objects I'm
querying and A_f0 the collection of childs of A. Then the outer join is
wrong. It should be:
A.id *= A_f0.parent
I realized that the _manyKey entry in the FieldDescriptor of "childs" is
null. How is that possible?
My mapping.xml looks like this:
<class name="A" identity="id" cache-type="none" key-generator="IDENTITY">
<map-to table="A"/>
<field name="childs" type="A" collection="collection" lazy="true">
<sql many-key="parent"/>
</field>
<field name="id" type="integer">
<sql name="id" type="integer"/>
</field>
<field name="parent" type="de.lmu.ip.search.categories.ConManCategory">
<sql name="parent" />
</field>
</class>
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