NHibernate: how to enable lazy loading on one-to-one mapping

Question

One-to-one relations within nhibernate can be lazyloaded either "false" or "proxy". I was wondering if anyone knows a way to do a lazy one-to-one mapping.

I worked out a hack to achieve the same result by using a lazy set mapped to a private field, and having the public property return the first result of that set. It works, but isn't the cleanest code...

Thanks in advance!

Answer 1

Lazy loading of one-to-one isn't supported unless the association is mandatory. See here for the reasoning.

It boils down to the fact that in order to decide if the other side of the relationship exists (N)Hibernate has to go to the database. Since you've already taken the database hit, you might as well load the full object.

While there are cases where hitting the DB just to see if the related object exists without actually loading the object makes sense (if the related object is very "heavy"), it isn't currently supported in NHibernate.

Answer 2

As far as I know, there isn't a non-hacky way to lazy load a one-to-one. I hope I'm wrong, but last time I checked it was the case.

Answer 3

There is way thought. It's described here in details :

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="NHibernateTest" namespace="NHibernateTest">
  <class name="Person" >
    <id name="PersonID" type="Int32">
      <generator class="identity" />
    </id>
    <property name="LastName" type="String" length="50" />
    <property name="FirstName" type="String" length="50" />
    <many-to-one name="Photo" class="PersonPhoto" />
  </class>

  <class name="PersonPhoto">
    <id name="PersonID" type="Int32">
      <generator class="foreign">
        <param name="property">Owner</param>
      </generator>
    </id>
    <property name="Photo" type="BinaryBlob" />
    <one-to-one name="Owner" class="Person" constrained="true" />
  </class>
</hibernate-mapping> 

Answer 4

I tried the example used by Artem Tikhomirov above. I kept getting an error that the Photo column does not exist. After looking at this, I figured out that the mapping was off a little. When I changed the many-to-one mapping to specify the column name like this:

many-to-one name="Photo" column="PersonID" class="PersonPhoto" unique="true"

I got it to work. I hope this helps someone :o)

Answer 5

After reading the answers here, I´ve manage to get it to work. I´m just going to add this example because I´m using a One to One relation with Constrained= False and because it´s a Mapping by Code Example

Two Classes:

public class Pedido:BaseModel
{
    public virtual BuscaBase Busca { get; set; }
}

public class BuscaBase : BaseModel
    {       
        public virtual Pedido Pedido { get; set; }
    }

Mappings:

public class PedidoMap : ClassMapping<Pedido>
{
    public PedidoMap()
    {
         Id(x => x.Id, x => x.Generator(Generators.Identity));            

         ManyToOne(x => x.Busca, m => 
         { 
             m.Cascade(Cascade.DeleteOrphans);
             m.NotNullable(true); m.Unique(true);
             m.Class(typeof(BuscaBase));
         });    
    }
}

public class BuscaBaseMap : ClassMapping<BuscaBase>
{
    public BuscaBaseMap()
    {            
        Id(x => x.Id, x => x.Generator(Generators.Sequence, g => g.Params(new { sequence = "buscaefetuada_id_seq" })));

        OneToOne(x => x.Pedido, m =>
        {
            m.Lazy(LazyRelation.NoProxy);
            m.Constrained(false);
            m.Cascade(Cascade.None);
            m.Class(typeof(Pedido));
        });            
    }
}

Note: I was Using for the one-to-one mapping m.PropertyReference(typeof(Pedido).GetProperty("Busca")); but this does't work for the lazy loading. You have to specify the relation using the Class

A quick brief about the Constrained = False used in here, the "Pedido" object might not exist in "BuscaBase" object.

Answer 6

What worked for me is the following (very similar to @Daniel) but I found that it was necessary to specify the LazyRelation.NoProxy on both ends of the mapping.

  public class Person
    {
        public virtual int Id { get; set; }

        public virtual string Name { get; set; }

        public virtual PersonDetails Details { get; set; }

        public class PersonMap : ClassMapping<Person>
        {
            public PersonMap()
            {
                Id(x => x.Id, m =>
                {
                    m.Generator(Generators.Native);
                });

                 
                Property(x => x.Name);

                OneToOne(x => x.Details, m =>
                { 
                    m.Lazy(LazyRelation.NoProxy);
                    m.Cascade(Cascade.Persist); 
                });
            }
        }
    }

    public class PersonDetails
    {
        public virtual int Id { get; set; }

        public virtual string ExtraDetails { get; set; }

        public virtual Person Person { get; set; }

        public class PersonDetailsMap : ClassMapping<PersonDetails>
        {
            public PersonDetailsMap()
            {

                Id(x => x.Id, m =>
                {
                    m.Generator(Generators.Native);
                }); 

                Property(x => x.ExtraDetails);
                
                ManyToOne(x => x.Person, m =>
                {
                    m.Lazy(LazyRelation.NoProxy);
                    m.Unique(true);
                    m.NotNullable(true); 
                });
            }
        }
    }


using var session = NhHelper.OpenSession();

var person1 = new Person();
person1.Name = "A";

var person1Details = new PersonDetails();
person1Details.ExtraDetails = "A details"; 

person1.Details = person1Details;
person1Details.Person = person1;

session.Save(person1);
//because of PersonMapping's Cascade.Persist it is not necessary to manually save person1Details object.
 

using var session = NhHelper.OpenSession();
foreach(var person in session.Query<Person>()) {
  Console.WriteLine(person.Name);  //<-- does not load PersonDetails unless it's property is accessed
}

• NHibernate 5.3.5
• Npgsql 5.0.3 (Postgresql Db).