Question

Does anyone know how the object can be made so that it does have something in its identity field – so that i can then use this to get the hash code.

Answer 1

One option is to use Guid as id. This way you don’t need database round trip to get identity value.

    <id name="_id" column="PersonId" >
        <generator class="assigned" />
    </id>

Or

Id(x => x.Id)
  .Column("PersonId")
  .GeneratedBy.Assigned();

‘PersonId’ column type should be uniqueidentifier if you use SQL Server. Then when you create object you can use Guid.NewGuid() to generate new id.

public class Person {
    private Guid _id;

    public Person() {
        _id = Guid.NewGuid();
    }

    public Guid Id {
        get { return _id; }
    }
}

The advantage of this approach is that your code is more decoupled from database. This will be very helpful during tests and if you need to support ‘disconnected’ scenarios. Potential disadvantage is the performance during joins (google “guid vs int primary key”). See this for additional information on id generators in NHibernate.

As a side note, you may want to rethink your design and implement your Equals and GetHashCode using business key. You will not need Guid identity in this case. From Don’t Let Hibernate Steal Your Identity:

“We recommend implementing equals() and hashCode() using Business key
equality. Business key equality means that the equals() method
compares only the properties that form the business key, a key that
would identify our instance in the real world (a natural candidate
key)” (Hibernate Reference Documentation v. 3.1.1).