NHibernate <one-to-many>

Question

How can I map these two classes?

public class LDAVehicle 
{
    public virtual int VehicleId { get; set; }
    public virtual string ChassisSeries { get; set; }
    public virtual string ChassisNumber { get; set; }

    public virtual List<LDAReading> Readings { get; set; }
}

public class LDAReading 
{
    public virtual int ReadingId { get; set; }
    public virtual DateTime IncomingDate { get; set; }
    public virtual DateTime ReadoutDate { get; set; }
    public virtual string Sender { get; set; }

    public virtual LDAVehicle Vehicle { get; set; }
}

Accually I have this xml:

<class name="LDAVehicle" table="Vehicle" schema="dbo" lazy="false">
    <id name="VehicleId">
        <column name="VehicleId" />
        <generator class="native"></generator>
    </id>
    <property name="ChassisSeries" column="ChassisSeries" not-null="false" />
    <property name="ChassisNumber" column="ChassisNumber" not-null="false" />

    <set name="Readings" table="Reading" fetch="join">
        <key foreign-key="VehicleId" />
        <one-to-many class="LDAReading" />
    </set>
</class>

<class name="LDAReading" table="Reading" schema="dbo" lazy="false">
    <id name="ReadingId">
        <column name="ReadingId" />
        <generator class="native"></generator>
    </id>
    <property name="IncomingDate" column="IncomingDate" not-null="true" />
    <property name="ReadoutDate" column="ReadoutDate" not-null="true" />
    <property name="Sender" column="Sender" not-null="false" />
    <many-to-one name="Vehicle" class="LDAVehicle" fetch="select" column="VehicleId" />
</class>

But I get an error:

An association from the table Reading refers to an unmapped class

Answer 1

There are few improvements I would made to make it running

1) C#

If we map collections, they MUST be represented as interfaces, so NHibernate will be able to inject proxies... own implementation

public class LDAVehicle 
{
    public virtual int VehicleId { get; set; }
    public virtual string ChassisSeries { get; set; }
    public virtual string ChassisNumber { get; set; }

    //public virtual List<LDAReading> Readings { get; set; }
    public virtual IList<LDAReading> Readings { get; set; }
}

public class LDAReading 
{
    public virtual int ReadingId { get; set; }
    public virtual DateTime IncomingDate { get; set; }
    public virtual DateTime ReadoutDate { get; set; }
    public virtual string Sender { get; set; }

    public virtual LDAVehicle Vehicle { get; set; }
}

2) hbm

For mapping collection I would use bag (until NHibernate 4 is released with native use of System stuff, we would need to use iesi collection ... so <bag> is good enough now) .

Also if propery name and column are same, we do not have to mention column.

Finally, all mappings are strongly LAZY now. This is simply the way which to go

<class name="LDAVehicle" table="Vehicle" schema="dbo" lazy="true"> // should be lazy
    <id name="VehicleId" column="VehicleId" generator="native" />  // simple id def

    <property name="ChassisSeries" not-null="false" /> // name is enough
    <property name="ChassisNumber" not-null="false" /> // if column is the same

    // IList and <bag> go work together
    // also profit from lazy loading, 
    //  batch size fetching and inverse mapping
    <bag name="Readings" lazy="true" inverse="true" 
                         batch-size="25" cascade="all-delete-orphan"> 
        <key column="VehicleId" />
        <one-to-many class="LDAReading" />
    </bag>
</class>

<class name="LDAReading" table="Reading" schema="dbo" lazy="true">
    <id name="ReadingId" generator="native" />

    <property name="IncomingDate" not-null="true" />
    <property name="ReadoutDate"  not-null="true" />
    <property name="Sender"       not-null="false" />

    <many-to-one name="Vehicle" class="LDAVehicle" column="VehicleId" />
</class>

3) be sure that both mapping are Emebedded Resources . If one of these is not found... not embedded... the exception as in the question is thrown... Also, both files should/must have suffix .hbm.xml

More reading:

• inverse mapping
• Ayende - NHibernate is lazy, just live with it
• batch size fetching