Question

Given a Team -> Athlete relationship and querying all athletes. What
am I misunderstanding about fetch="Join"? Should this mapping cause
the Team to be loaded via a join? When iterating the athletes, it
still lazy loads the Team.

public class AthleteMap : ClassMapping<Athlete>
{
        public AthleteMap()
        {
            ManyToOne(a => a.Team, o =>
                                       {
                                           o.Fetch(FetchKind.Join);
                                           o.Lazy(LazyRelation.NoLazy);
                                       }
                );    
        }    
}

Which produces this HBM:

<class name="Athlete" table="Athletes">
    <id name="Id" type="Int32" />
    <property name="FirstName" />
    <property name="LastName" />
    <many-to-one name="Team" fetch="join" lazy="false" />
    <property name="Created" />
</class>

iterating:

var session = factory.OpenSession();

 foreach (var athlete in session.Query<Athlete>())
     Console.WriteLine("{0} {1}", athlete.FirstName, athlete.Team.Name); 

Answer 1

The NHibernate Linq Query doesn’t use the fetch strategy of the mapping. You have to Fetch() in your linq query like this.

var session = factory.OpenSession();

foreach (var athlete in session.Query<Athlete>().Fetch(x => x.Team))
   Console.WriteLine("{0} {1}", athlete.FirstName, athlete.Team.Name); 

The fetch strategy defined in the mapping document affects:

• retrieval via Get() or Load()
• retrieval that happens implicitly when an association is navigated
• ICriteria queries
• HQL queries if subselect fetching is used

source:performance-fetching