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How to create mapping for additional class inside some class?

I have two classes: Order and User
Order has User class inside of it:
[DataMember]
public virtual User User { get; set; }
I have NHibernate mapping for both of them:
For User:
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
assembly="Purchasing" namespace="Purchasing.Other">
<class name="User" table="tUser">
<id name="RID">
<column name="RID" sql-type="bigint"/>
<generator class="native"/>
</id>
<property name="Created"/>
<property name="Modified"/>
<property name="UserName"/>
<property name="Email"/>
<property name="ExternalUserId"/>
</class>
</hibernate-mapping>
And I tried to add User class to Order mapping, BUT looks like something is wrong:
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
assembly="Purchasing" namespace="Purchasing.Other">
<class name="Order" table="tOrder">
<id name="Id">
<column name="OrderId" sql-type="bigint"/>
<generator class="native" />
</id>
<property name="DataOwnerId" />
<property name="UserId" />
<property name="OrderNo"/>
<property name="FirstTaken"/>
<property name="DateRequired" />
<property name="ExternalOrderId" />
<many-to-one name="User" class="User" property-ref="ExternalUserId">
<column name="UserId" not-null="false"/>
</many-to-one>
</class>
</hibernate-mapping>
I have a situation, when, writing Order in tOrder database, I also need to write its User to tUser database.
But looks like it didn't work. (Order mapping without User map works fine).
How in this situation mapping should look correctly, and what am I missing?
P.S Sorry for my bad English.

If we want to persist User with its Order, we can use cascading setting:
5.1.10. many-to-one
An ordinary association to another persistent class is declared using a many-to-one element. The relational model is a many-to-one association. (It's really just an object reference.)
<many-to-one
name="PropertyName" (1)
...
cascade="all|none|save-update|delete" (4)
...
/>
(4) cascade (optional): Specifies which operations should be cascaded from the parent object to the associated object.
So the mapping could be like this:
<many-to-one name="User" class="User"
property-ref="ExternalUserId"
cascade="save-update"
column="UserId" not-null="false"
/>
Now, we only have to be sure, that we try to find a user (it could be existing)
var user = session.Get<User>(userId);
if(user == null)
{
user = new User { ... };
}
And then we can assing that user to the Order
var order = new Order { ... };
order.User = user;
and calling session to persist Order - user will be as well:
session.SaveOrUpdae(order);
NOTE: if you can reference user by its native ID ... it would be better. Mapping then would look like:
<many-to-one name="User" class="User"
cascade="save-update"
column="RID" not-null="false"
/>
Mapping with property-ref should be used exceptionally

nhibernate subclass - zero guid in not used column instead of null

I have an issue with a two subclasses using the same table and same fields. Even though the generated table has this fields as nullable uniqueidentifiers, when I save the one record that does not use the third column, then the created field has a value of "000000000-0000-0000-0000-000000000000" instead of NULL. Other parts of application require this place to be null. Here's the code. What's wrong?
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
assembly=".."
namespace="...">
<class name="TEST" table="..." discriminator-value="not null">
<id name="Id" column="tID">
<generator class="assigned" />
</id>
<discriminator column="iTestTypeID" type="Int32" not-null="true"/>
<property name="TotalScore" column="fTotalScore"/>
<property name="IsTimedOut" column="bTimedOut"/>
</hibernate-mapping>
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
assembly=".."
namespace="...">
<subclass name="TestA" extends="TEST" discriminator-value="3">
<many-to-one name="Department" column="uTestCriteria01" cascade="save-update" not-null="false" />
<many-to-one name="Building" column="uTestCriteria02" cascade="save-update" not-null="false" />
</subclass>
</hibernate-mapping>
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
assembly=".."
namespace="...">
<subclass name="TestB" extends="TEST" discriminator-value="3">
<many-to-one name="Building" column="uTestCriteria01" cascade="save-update" not-null="false" />
<many-to-one name="Floor" column="uTestCriteria02" cascade="save-update" not-null="false" />
<many-to-one name="Room" column="uTestCriteria03" cascade="save-update" not-null="false" />
</subclass>
</hibernate-mapping>

My guess would be that your objects aren't using nullable types.
Do you have the types defined like this:
public class A
{
public Guid? field { get; set; }
}
Take a look at this StackOverflow article for an explanation:
How can I default a parameter to Guid.Empty in C#?
Also in your question I would list the specific field(s) you are talking about so there is no confusion and post the code for your objects that are tied to this mapping.

NHibernate - cascade delete is not working

I use NHibernate 3.2 with MS SQL Server 2008 R2
I have the fallowing mappings
<class name="LocalizedProperty" table="LocalizedProperty">
<cache usage="read-write"/>
<id name="Id" column="Id">
<generator class="guid.comb"/>
</id>
<property name="CultureName" not-null="true"/>
<property name="PropertyName" not-null="true"/>
<property name="PropertyValue" not-null="true"/>
<any id-type="Guid" name="Entity">
<column name="LocalizedEntityClass" not-null="true"/>
<column name="EntityId" not-null="true"/>
</any>
</class>
And this one has a Reference to LocalizedProperty:
<class name="CommunicationType" table="CommunicationType" lazy="false" >
...
<set name="LocalizedProperties" where="LocalizedEntityClass = 'Prayon.Entities.CommunicationType'" cascade="delete">
<key column="EntityId" foreign-key="none" />
<one-to-many class="LocalizedProperty" />
</set>
</class>
My Problem is, when I delete an entity of CommunicationType, NHibernate is executing the fallowing update-statement for LocalizedProperty
UPDATE LocalizedProperty SET EntityId = null WHERE EntityId = #p0 AND (LocalizedEntityClass = 'Prayon.Entities.CommunicationType')
Instead of a Delete-Statement.
Does someone see, what is wrong?

To delete child elements of a table you need to specify cascade="all-delete-orphan".
<bag name="TableClassName" table="TableClassName" cascade="all-delete-orphan" >
<key column="PrimaryKey"/>
<one-to-many class="NameSpace.TableClassName" />

If you put a all-delete-orphan the problem is the all part. It will cascade all the actions, not just the delete.

Need access to table with string based primary key in nHibernate

I am trying to map a SQL Server database with nHibernate that is full of tables with varchar primary keys that are generated by external software and I need update/read (no insert) access.
I cannot find a way to get past the following error:
XXXX.Tests.GMCRepository_Fixture.Can_get_existing_GMC_by_parameter'
failed: NHibernate.MappingException :
XXXX.Domain.Mappings.GMC2.hbm.xml(4,6): XML validation error: The element 'class' in namespace 'urn:nhibernate-mapping-2.2' has invalid child element 'property' in namespace 'urn:nhibernate-mapping-2.2'. List of possible elements expected: 'meta, subselect, cache, synchronize, comment, tuplizer, id, composite-id' in namespace 'urn:nhibernate- mapping-2.2'.
Research has suggested this error is relating to there not being a valid primary key (id) defined.
Mapping XML looks like:
<?xml version="1.0" encoding="utf-8"?>
<hibernate-mapping assembly="XXXX.Domain" namespace="XXXX.Domain" xmlns="urn:nhibernate-mapping-2.2" schema="GM.dbo">
<class name="GMC2" table="C2" lazy="true" >
<property name="PARAMETER">
<column name="PARAMETER" sql-type="varchar" not-null="true" />
</property>
...
<id name="Recid">
<column name="recid" sql-type="varchar" not-null="true" />
</id>
</class>
</hibernate-mapping>
Thanks for your help!

I believe that the convention is to have the ID mapping as the first thing under the class declaration.
Also as part of the Id mapping you need to specify the Generator of the ID. In your case I think you will need the assigned generator added to your ID mapping. Your class mapping will look something like this.
<?xml version="1.0" encoding="utf-8"?>
<hibernate-mapping assembly="XXXXCRMAPI.Domain" namespace="XXXXCRMAPI.Domain" xmlns="urn:nhibernate-mapping-2.2" schema="GoldMine.dbo">
<class name="GMContact2" table="CONTACT2" lazy="true" >
<id name="Recid">
<generator type="assigned" />
<column name="recid" sql-type="varchar" not-null="true" />
</id>
<property name="Accountno">
<column name="ACCOUNTNO" sql-type="varchar" not-null="true" />
</property>
...
</class>
</hibernate-mapping>

NHibernate Criteria: Add a constraint for property type

I have a program with a base class (Message) from which many other classes derivate (notably Article and Comment). I would like to create a criteria that will select all the Comments for which the RootMessage property is of type Article (I have some other type of potential RootMessages). But I can't seem to find how to add that kind of constraint in a criteria.
For now my code is this:
public virtual ICollection<Comment> GetCommentsByArticle(ISession session)
{
var c = session.CreateCriteria(typeof(Comment))
.Add(Expression.Eq("Author", this))
.AddOrder(Order.Desc("CreationDate"))
.SetMaxResults(20);
var commentList = c.List<Comment>();
return commentList;
}
But of course this returns every single comment of this Author. What I want is every single comment of this Author that have a RootMessage of the Article type.
These are the mappings.
Message:
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
assembly="FolkeLib"
namespace="Folke.Code.Domain">
<class name="Message" discriminator-value="0">
<id name="Id">
<generator class="native"></generator>
</id>
<discriminator column="MessageType" type="Int32"/>
<timestamp column="ModificationDate" name="ModificationDate"/>
<property name="CreationDate" index="ArticleCreationDate"/>
<many-to-one name="Author"/>
<property name="Text" type="StringClob">
<column name="Text" sql-type="text"/>
</property>
<property name="Locked"/>
<many-to-one name="Locker" not-null="false"/>
<many-to-one name="RootMessage" not-null="false"/>
<many-to-one name="ParentMessage" not-null="false"/>
<property name="Score"/>
<many-to-one name="Site" not-null="true"/>
Article:
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
assembly="FolkeLib"
namespace="Folke.Code.Domain">
<subclass name="Article" extends="Message" discriminator-value="1">
<property name="Title"/>
<set name="TagSet" lazy="true" table="MessageTag">
<key column="MessageId"/>
<many-to-many class="Tag" column="TagId"/>
</set>
<set name="ImageSet" lazy="true" table="MessageImage">
<key column="MessageId"/>
<many-to-many class="Image" column="ImageId"/>
</set>
<many-to-one name="Forum"/>
</subclass>
</hibernate-mapping>
Comment (class is pretty much empty for now):
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
assembly="FolkeLib"
namespace="Folke.Code.Domain">
<subclass name="Comment" extends="Message" discriminator-value="2">
</subclass>
</hibernate-mapping>
Any help appreciated!

This would be easier to answer if you posted your complete mappings, but I believe something along the lines of:
.CreateAlias("RootMessage", "rm")
.Add(Expression.Eq("rm.class",typeof(Article)))
Is what you're looking for.