Question

While trying to create a Bi-directional one-to-one mapping in NHibernate, I found that, I am unable to have the Reference of the objects recursively.

For example: suppose I have one-to-one relationships between Person and Address.

then after executing the following code,

class Person
{
    ... ...
    public Address Address { get;set; }
}

class Address
{
    ... ...
    public Person Person {get;set;}
}

Repository<Person> rep = new Repository<Person>();
Person p = rep.Get<Person>(1);

I need to have a non-null value from p.Address.Person. I.e. the same person with an ID of 1.

But the property is returning a null-value.

What should I look for to solve the problem?

My database tables are like this:

Address {ID, Desc}
Person {ID, Name, AddressID}

Person.hbm.xml

<?xml version="1.0" encoding="utf-8" ?>

<hibernate-mapping
    xmlns="urn:nhibernate-mapping-2.2"
     default-access="property"
    >
  <class name="NHibernate__BiDirectional__One_To_One.BO.Person, NHibernate__BiDirectional__One_To_One.BO" 
         table="Person">
    <id name="ID">
      <generator class="native" />
    </id>
    <property name="Name"/>

    <many-to-one
        name="Address"
        class="NHibernate__BiDirectional__One_To_One.BO.Address, NHibernate__BiDirectional__One_To_One.BO"
        column="AddressID" 
        cascade="all" 
        unique="true" />

  </class>
</hibernate-mapping>

Address.hbm.xml

<?xml version="1.0" encoding="utf-8" ?>

<hibernate-mapping
  xmlns="urn:nhibernate-mapping-2.2"
   default-access="property"
  >
  <class name="NHibernate__BiDirectional__One_To_One.BO.Address, NHibernate__BiDirectional__One_To_One.BO" 
         table="Address">
    <id name="ID" >
      <generator class="native" />
    </id>
    <property name="Desc"/>      
    <one-to-one
        name="Person"
        class="NHibernate__BiDirectional__One_To_One.BO.Person, NHibernate__BiDirectional__One_To_One.BO"
        />
  </class>
</hibernate-mapping>

I am also getting an error:

could not load an entity: [NHibernate__BiDirectional__One_To_One.BO.Person#1][SQ
L: SELECT person0_.ID as ID0_1_, person0_.Name as Name0_1_, address1_.ID as ID1_
0_, address1_.Desc as Desc1_0_, address1_.AddressID as AddressID1_0_ FROM Person
 person0_ left outer join Address address1_ on person0_.ID=address1_.AddressID W
HERE person0_.ID=?]
Incorrect syntax near the keyword 'Desc'.

Answer 1

There are two varieties of one-to-one association:

• primary key associations

• unique foreign key associations

Primary key associations don’t need an extra table column; if two rows are related by the association then the
two table rows share the same primary key value. So if you want two objects to be related by a primary key association,
you must make sure that they are assigned the same identifier value!
For a primary key association, add the following mappings to Employee and Person, respectively.

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

Now we must ensure that the primary keys of related rows in the PERSON and EMPLOYEE tables are equal.

We use a special NHibernate identifier generation strategy called foreign:

<class name="Person" table="PERSON">
<id name="Id" column="PERSON_ID">
<generator class="foreign">
<param name="property">Employee</param>
</generator>
</id>
...
<one-to-one name="Employee"
class="Employee"
constrained="true"/>
</class>

A newly saved instance of Person is then assigned the same primar key value as the Employee instance refered
with the Employee property of that Person.
Alternatively, a foreign key with a unique constraint, from Employee to Person, may be expressed as:

<many-to-one name="Person" class="Person" column="PERSON_ID" unique="true"/>

And this association may be made bidirectional by adding the following to the Person mapping:

<one-to-one name="Employee" class="Employee" property-ref="Person"/>

Source: Chapter 5. Basic O/R Mapping – 5.1.12. one-to-one

Have a look at this

Hibernate Community • View topic – one-to-one with foreign key on child table.