Hi,
I'm still a relative newcomer to NHibernate, but I've been puzzling
with this issue for a while and haven't come up with a reasonable
answer.
I'm using NH 2.0.1 GA with SQL 2005 db.
Basically the issue is that in certain circumstances, NHibernate is
ignoring the fetch strategy on a one-to-many collection that I have
specified as "join" and instead using a single select.
To illustrate this I knocked up a simple example (excuse the dodgy
object model!)...
My mapping looks like this...
<class name="Parent">
<id name="ParentID">
<generator class="identity" />
</id>
<property name="FirstName" />
<property name="LastName" />
<bag name="Children">
<key column="ParentID"/>
<one-to-many class="Child"/>
</bag>
</class>
<class name="Child">
<id name="ChildID">
<generator class="identity" />
</id>
<property name="FirstName" />
<property name="LastName" />
<bag name="Grandchildren" fetch="join">
<key column="ChildID"/>
<one-to-many class="Grandchild"/>
</bag>
<many-to-one name="Parent" class="Parent" column="ParentID"/>
</class>
<class name="Grandchild">
<id name="GrandchildID">
<generator class="identity" />
</id>
<property name="FirstName" />
<property name="LastName" />
<many-to-one name="Parent" class="Child" column="ChildID"/>
</class>
---------------------------
So my two scenarios are this...
1)
using (ISession session = NHibernateHelper.OpenSession())
{
Child child = session.Get<Child>(1);
Grandchild gc = child.Grandchildren[0];
}
This executes fine and leads to a single SQL statement as expected -
NHibernate: SELECT child0_.ChildID as ChildID1_1_, child0_.FirstName
as FirstName1_1_, child0_.LastName as LastName1_1_, child0_.ParentID
as ParentID1_1_, grandchild1_.ChildID as ChildID3_,
grandchild1_.GrandchildID as Grandchi1_3_, grandchild1_.GrandchildID
as Grandchi1_2_0_, grandchild1_.FirstName as FirstName2_0_,
grandchild1_.LastName as LastName2_0_, grandchild1_.ChildID as
ChildID2_0_ FROM Child child0_ left outer join Grandchild grandchild1_
on child0_.ChildID=grandchild1_.ChildID WHERE child0_.child...@p0; @p0
= '1'
2)
using (ISession session = NHibernateHelper.OpenSession())
{
IList<Parent> parents = session.CreateCriteria(typeof
(Parent)).List<Parent>();
Child child = parents[0].Children[0];
Grandchild gc = child.Grandchildren[0];
}
This executes 3 selects (rather than the 2 I would expect)
NHibernate: SELECT this_.ParentID as ParentID0_0_, this_.FirstName as
FirstName0_0_, this_.LastName as LastName0_0_ FROM Parent this_
NHibernate: SELECT children0_.ParentID as ParentID1_,
children0_.ChildID as ChildID1_, children0_.ChildID as ChildID1_0_,
children0_.FirstName as FirstName1_0_, children0_.LastName as
LastName1_0_, children0_.ParentID as ParentID1_0_ FROM Child
children0_ WHERE children0_.parent...@p0; @p0 = '1'
NHibernate: SELECT grandchild0_.ChildID as ChildID1_,
grandchild0_.GrandchildID as Grandchi1_1_, grandchild0_.GrandchildID
as Grandchi1_2_0_, grandchild0_.FirstName as FirstName2_0_,
grandchild0_.LastName as LastName2_0_, grandchild0_.ChildID as
ChildID2_0_ FROM Grandchild grandchild0_ WHERE
grandchild0_.child...@p0; @p0 = '1'
Can anyone explain why in the second example, it is still performing
an extra select to get the Grandchildren rather than joining as it did
in the first example? Maybe I'm doing something stupid, but it's got
me stumped!
Thanks in advance
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