Question

I have two entity classes: Person and Position.

public class Person
{
  public virtual int Id { get; set; }
  public virtual string Name { get; set; }
  public virtual Position Position { get; set; }
}

public class Position
{
  public virtual int Id { get; set; }
  public virtual string Code { get; set; }
}

Person has a one-to-many relationship with Position. I also have a DTO PersonDto that brings the person data from the client.

public class PersonDto
{
  public string Name { get; set; }
  public int PositionId { get; set; }
}

After I get the DTO I have to create a new Person object and persist it. Usually it looks like this:

var person = new Person();
CopyFromDto(person, personDto);
var position = positionRepository.GetById(personDto.PositionId);
person.Position = position;
personRepository.Save(person);

I wonder if I could get rid of this line:

var position = positionRepository.GetById(personDto.PositionId);

The reason is that it produces an unneccessary SQL query that fetches some position data. Is it possible to somehow provide NHibernate with the position id and save a person row with it?

Thanks in advance.

Answer 1

Use ISession.Load:

The Load() methods of ISession give you a way to retrieve a persistent
instance if you already know its identifier.

Note that Load() will throw an unrecoverable exception if there is no
matching database row. If the class is mapped with a proxy, Load()
returns an object that is an uninitialized proxy and does not actually
hit the database until you invoke a method of the object. This
behaviour is very useful if you wish to create an association to an
object without actually loading it from the database.

https://nhibernate.info/doc/nh/en/index.html#manipulatingdata-loading