nhibernate Composite-id，不存在键多对一记录

Question

i have old legacy DB which has dead links in their tables. I have class mapped in nhibernate like this:

<class name="Visible" table="table_visible">
    <composite-id>
        <key-many-to-one column="object_id" name="ObjectA" />
        <key-many-to-one column="sub_object_id" name="SubObject" />
    </composite-id>
    <property column="visible" name="VisibleRow" />
</class>

and:

public class Visible
{
    public virtual ObjectAClass ObjectA { get; set; }
    public virtual SubObjectClass SubObject { get; set; }
    public virtual bool VisibleRow { get; set; }

    public override bool Equals(object obj)
    {
        var other = ((Visible)obj);
        return this.ObjectA.Equals(other.ObjectA) && this.SubObject.Equals(other.SubObject);
    }

    public override int GetHashCode()
    {
        return this.ObjectA.GetHashCode() + (this.SubObject != null? this.SubObject.GetHashCode(): 0);
    }
}

Now all works fine when all joins in database are correct, but when i find such sub_object_id which doesnt have entity, nhibernate throws me error

No row with the given identifier exists:[SubObject#123]

Is there a way to map composite key so that when its subentity is not found, the whole entity wouldnt be loaded (like with inner join)?

NHibernate v2.0.50727

Answer 1

Following Daniel Schilling idea of fetch Visible entities with a where exists sub-query, found that there is loader element available in mappings.

<class name="ObjectA" table="table_object">
    .........   
    <set name="VisibleList" cascade="all" lazy="false" inverse="true">
        <key column="object_id" />
        <one-to-many class="Visible" />
        <loader query-ref="valid_entities"/>
    </set>

</class>
<sql-query name="valid_entities">
    <load-collection alias="v" role="ObjectA.VisibleList"/>
    SELECT {v.*}
    FROM table_visible v
    INNER JOIN table_sub_entities e ON e.sub_entity_id=v.sub_entity_id
    WHERE v.object_id=?
</sql-query>

And nothing else needed to be changed.

Answer 2

<key-many-to-one column="sub_object_id" name="SubObject" not-found="ignore" />

... may be helpful. From the NHibernate Documentation ...

ignore will treat a missing row as a null association

Please be aware of the performance penalty associated with using this option. Whenever NHibernate fetches a Visible entity, it will also have to fetch SubObject . If you don't go ahead and fetch it in your query, this means that NHibernate will be issuing lots of lazy loads.

This doesn't meet your "when its sub-entity is not found, the whole entity wouldn't be loaded" goal. Instead NHibernate would give you an entity with a null sub-entity. If you want that inner-join-like behavior, then I think you would need to fetch your Visible entities with a where exists sub-query to make sure the SubObject actually exists.

The best option would be to fix the data in the database and add a foreign key constraint.

I just ran across this: Relations with not-found="ignore" . I promise I'm not copying Ricci's content - I'm writing this from my own experience.