NHibernate hasOne lazy load

Question

I have a parent-child relation, with just one child on Sql Server. The child contains fat blob data I won't load. The child is optional and it must depend on the lifecycle of the parent. So the foreign key on the child, point to the parent and is unique.

I can use the official example on Hibernate reference.

public class Phone
{
    public virtual long Id { get; set; }

    public virtual string Number { get; set; }

    public virtual PhoneDetails Details { get; set; }
}

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

    public virtual Phone Phone { get; set; }

    public virtual string Provider { get; set; }
}

Phone details must depend on parent lifecycle on the database. I can use a one-to-one relation on sql with a unique foreign key on the child, or many-to-one relation with a simple foreign key (as in example) plus a unique constraint.

CREATE TABLE Phone (
    id BIGINT IDENTITY PRIMARY KEY,
    number VARCHAR(255)
)

CREATE TABLE PhoneDetails (
    id BIGINT IDENTITY PRIMARY KEY,
    phone_id BIGINT UNIQUE FOREIGN KEY REFERENCES dbo.Phone(id),
    provider VARCHAR(255)        
)

And this is fine i think:

So I have a Phone, this can live without details, and when I need I can add only one detail to complete my object with other details I don't want to load always.

How can I map these classes on NHibernate 5? It seems that i need to use HasOne, in a bidirectional way, but in that case I cannot use lazy load so every simple query on the Phone, will lead to a join and select of all the fields of the details.

I use the details table to store huge metadata that I must not need at 90% of time, but now everywhere I try to load a Phone, the query load also the huge details, this is really bad.

So in which way can I map this relation?

A parent, and a single child containing not useful fat data. In sql I think the structure is ok, because I don't want more children, and I want for them to live under the parent (so FK on child).

It is impossible that is not managed this type of relation, where am I wrong? database design? mapping?

This is the mapping I use:

  <class name="Phone" table="Phone">
    <id name="Id">
      <generator class="native"/>
    </id>
    <property name="Number"/>
    <one-to-one name="PhoneDetails" class="PhoneDetails"/>
  </class>

  <class name="PhoneDetails" table="PhoneDetails">
    <id name="Id">
      <generator class="native"/>
    </id>
    <property name="Provider" />
    <many-to-one name="Phone" column="phone_id" unique="true"/>    
  </class>

I have also tried with the second options, to use a foreign\primary key on the relational model (so i removed the FK and used the same Id as PK\FK), with this mapping in the child:

   <one-to-one name="Phone" class="Phone" constrained="true" />

Answer 1

According to this SO answer, Lazy-Loading is not supported on non-mandatory one-to-one relationships.

So, in order to work around this, why don't you get rid of this one-to-one relationship ? If you do not need the details 90% of the time, I would consider removing the relation-ship and have a seperate repository method that just enables you to retrieve the PhoneDetails for a specified Phone.

To continue with your example, I would end up with this:

public class Phone
{
    public virtual long Id { get; set; }

    public virtual string Number { get; set; }
}

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

    public virtual Phone BelongsToPhone { get; set; }

    public virtual string Provider { get; set; }
}

public class PhoneRepository
{
      public Phone GetPhone( long id ){}

      public PhoneDetails GetPhoneDetails( Phone phone ){}
}

Answer 2

A nullable one-to-one cannot know without querying the associated table whether it exists or not, thus for loading its proxy it is required to query the associated table. From that point, it has up to known been consider in past discussions to be better to query the whole state rather than just the key, thus the eager load. -> You won't be able to enable lazy loading on a reference that is not mandatory. At least not with a standard solution.

If you want the relation to be able to lazy load the PhoneDetails you will have to make it mandatory / not nullable.

As an alternative you could mark the properties in the PhoneDetails as lazy="true". Therefore the table will be accessed and a proxy will be created for the PhoneDetail-object as soon as you load the parent entity but all properties marked as lazy="true" will only be loaded as soon as you access them. Please note that ALL properties with lazy="true" will be loaded as soon as you access one of them