Question

I have Canine and CanineHandler objects in my application. The CanineHandler object has a PersonID (which references a completely different database), an EffectiveDate (which specifies when a handler started with the canine), and a FK reference to the Canine (CanineID).

Given a specific PersonID, I want to find all canines they’re currently responsible for. The (simplified) query I’d use in SQL would be:

Select Canine.*
    from Canine
        inner join CanineHandler on(CanineHandler.CanineID=Canine.CanineID)
        inner join 
            (select CanineID,Max(EffectiveDate) MaxEffectiveDate
                from caninehandler
                group by CanineID) as CurrentHandler
            on(CurrentHandler.CanineID=CanineHandler.CanineID
                and CurrentHandler.MaxEffectiveDate=CanineHandler.EffectiveDate)
    where CanineHandler.HandlerPersonID=@PersonID

Edit: Added mapping files below:

<class name="CanineHandler" table="CanineHandler" schema="dbo">
    <id name="CanineHandlerID" type="Int32">
        <generator class="identity" />
    </id>
    <property name="EffectiveDate" type="DateTime" precision="16" not-null="true" />
    <property name="HandlerPersonID" type="Int64" precision="19" not-null="true" />
    <many-to-one name="Canine" class="Canine" column="CanineID" not-null="true" access="field.camelcase-underscore" />
</class>

<class name="Canine" table="Canine">
    <id name="CanineID" type="Int32">
        <generator class="identity" />
    </id>
    <property name="Name" type="String" length="64" not-null="true" />
    ...
    <set name="CanineHandlers" table="CanineHandler" inverse="true" order-by="EffectiveDate desc" cascade="save-update" access="field.camelcase-underscore">
        <key column="CanineID" />
        <one-to-many class="CanineHandler" />
    </set>
    <property name="IsDeleted" type="Boolean" not-null="true" />
</class>

I haven’t tried yet, but I’m guessing I could do this in HQL. I haven’t had to write anything in HQL yet, so I’ll have to tackle that eventually anyway, but my question is whether/how I can do this sub-query with the criterion/subqueries objects.

I got as far as creating the following detached criteria:

DetachedCriteria effectiveHandlers = DetachedCriteria.For<Canine>()
                .SetProjection(Projections.ProjectionList()
                    .Add(Projections.Max("EffectiveDate"),"MaxEffectiveDate")
                    .Add(Projections.GroupProperty("CanineID"),"handledCanineID")
                );

but I can’t figure out how to do the inner join. If I do this:

Session.CreateCriteria<Canine>()
    .CreateCriteria("CanineHandler", "handler", NHibernate.SqlCommand.JoinType.InnerJoin)
    .List<Canine>();

I get an error “could not resolve property: CanineHandler of: OPS.CanineApp.Model.Canine”. Obviously I’m missing something(s) but from the documentation I got the impression that should return a list of Canines that have handlers (possibly with duplicates). Until I can make this work, adding the subquery isn’t going to work…

I’ve found similar questions, such as Only get latest results using nHibernate but none of the answers really seem to apply with the kind of direct result I’m looking for.

Any help or suggestion is greatly appreciated.

Answer 1

Joining to a derived table, CurrentHandler in your example, won’t work in HQL the last time I checked. Try mapping a stored procedure that lets you write whatever SQL you like. Here’s what a mapped stored procedure looks like:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="S2.BP.NHSupport" namespace="S2.BP.Model">
  <sql-query name="spGoGetMyDogs" callable="true">
    <return-scalar column="PersonID" type="int" />
    exec spGoGetMyDogs @PersonID=:personID
  </sql-query>
</hibernate-mapping>

Then you can pass your PersonID parameter in and have NH map the results back to your objects with a transformer like so:

public IEnumerable<Canine> LetTheDogsOut(int personID) {
  return nhSession.GetNamedQuery("spGoGetMyDogs")
    .SetInt32("personID", personID)
    .SetResultTransformer(Transformers.AliasToBean(typeof(Canine)))
    .List<Canine>();
}