NHibernate - Relational Persistence for Idiomatic .NET

This tutorial explains a setup of NHibernate 1.0.2 within a Microsoft environment. The tools used in this tutorial are:

First, we have to create a new Web project. We use the name QuickStart, the project web virtual directory will http://localhost/QuickStart. In the project, add a reference to NHibernate.dll. Visual Studio will automatically copy the library and its dependencies to the project output directory. If you are using a database other than SQL Server, add a reference to the driver assembly to your project.

We now set up the database connection information for NHibernate. To do this, open the file Web.config automatically generated for your project and add configuration elements according to the listing below:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <!-- Add this element -->
    <configSections>
        <section
            name="hibernate-configuration"
            type="NHibernate.Cfg.ConfigurationSectionHandler, NHibernate"
        />
    </configSections>

    <!-- Add this element -->
    <hibernate-configuration xmlns="urn:nhibernate-configuration-2.0">
        <session-factory>
            <property name="dialect">NHibernate.Dialect.MsSql2000Dialect</property>
            <property name="connection.provider">NHibernate.Connection.DriverConnectionProvider</property>
            <property name="connection.connection_string">Server=(local);initial catalog=quickstart;Integrated Security=SSPI</property>
            
            <mapping assembly="QuickStart" />
        </session-factory>
    </hibernate-configuration>

    <!-- Leave the system.web section unchanged -->
    <system.web>
        ...
    </system.web>
</configuration>

The <configSections> element contains definitions of sections that follow and handlers to use to process their content. We declare the handler for the configuration section here. The <hibernate-configuration> section contains the configuration itself, telling NHibernate that we will use a Microsoft SQL Server 2000 database and connect to it through the specified connection string. The dialect is a required setting, databases differ in their interpretation of the SQL "standard". NHibernate will take care of the differences and comes bundled with dialects for several major commercial and open source databases.

An ISessionFactory is NHibernate's concept of a single datastore, multiple databases can be used by creating multiple XML configuration files and creating multiple Configuration and ISessionFactory objects in your application.

The last element of the <hibernate-configuration> section declares QuickStart as the name of an assembly containing class declarations and mapping files. The mapping files contain the metadata for the mapping of the POCO class to a database table (or multiple tables). We'll come back to mapping files soon. Let's write the POCO class first and then declare the mapping metadata for it.

NHibernate works best with the Plain Old CLR Objects (POCOs, sometimes called Plain Ordinary CLR Objects) programming model for persistent classes. A POCO has its data accessible through the standard .NET property mechanisms, shielding the internal representation from the publicly visible interface:

using System;

namespace QuickStart
{
    public class Cat
    {
        private string id;
        private string name;
        private char   sex;
        private float  weight;

        public Cat()
        {
        }

        public string Id
        {
            get { return id; }
            set { id = value; }
        }

        public string Name
        {
            get { return name; }
            set { name = value; }
        }

        public char Sex
        {
            get { return sex; }
            set { sex = value; }
        }

        public float Weight
        {
            get { return weight; }
            set { weight = value; }
        }
    }
}

NHibernate is not restricted in its usage of property types, all .NET types and primitives (like string, char and DateTime) can be mapped, including classes from the System.Collections namespace. You can map them as values, collections of values, or associations to other entities. The Id is a special property that represents the database identifier (primary key) of that class, it is highly recommended for entities like a Cat. NHibernate can use identifiers only internally, without having to declare them on the class, but we would lose some of the flexibility in our application architecture.

No special interface has to be implemented for persistent classes nor do we have to subclass from a special root persistent class. NHibernate also doesn't use any build time processing, such as IL manipulation, it relies solely on .NET reflection and runtime class enhancement (through Castle.DynamicProxy library). So, without any dependency in the POCO class on NHibernate, we can map it to a database table.

The Cat.hbm.xml mapping file contains the metadata required for the object/relational mapping. The metadata includes declaration of persistent classes and the mapping of properties (to columns and foreign key relationships to other entities) to database tables.

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0"
    namespace="QuickStart" assembly="QuickStart">

    <class name="Cat" table="Cat">

        <!-- A 32 hex character is our surrogate key. It's automatically
            generated by NHibernate with the UUID pattern. -->
        <id name="Id">
            <column name="CatId" sql-type="char(32)" not-null="true"/>
            <generator class="uuid.hex" />
        </id>

        <!-- A cat has to have a name, but it shouldn' be too long. -->
        <property name="Name">
            <column name="Name" length="16" not-null="true" />
        </property>
        <property name="Sex" />
        <property name="Weight" />
    </class>

</hibernate-mapping>

Every persistent class should have an identifer attribute (actually, only classes representing entities, not dependent value objects, which are mapped as components of an entity). This property is used to distinguish persistent objects: Two cats are equal if catA.Id.Equals(catB.Id) is true, this concept is called database identity. NHibernate comes bundled with various identifer generators for different scenarios (including native generators for database sequences, hi/lo identifier tables, and application assigned identifiers). We use the UUID generator (only recommended for testing, as integer surrogate keys generated by the database should be prefered) and also specify the column CatId of the table Cat for the NHibernate generated identifier value (as a primary key of the table).

All other properties of Cat are mapped to the same table. In the case of the Name property, we mapped it with an explicit database column declaration. This is especially useful when the database schema is automatically generated (as SQL DDL statements) from the mapping declaration with NHibernate's SchemaExport tool. All other properties are mapped using NHibernate's default settings, which is what you need most of the time. The table Cat in the database looks like this:

 Column |     Type     | Modifiers
--------+--------------+----------------------
 CatId  | char(32)     | not null, primary key
 Name   | nvarchar(16) | not null
 Sex    | nchar(1)     |
 Weight | real         |

You should now create the database and this table manually, and later read Chapter 15, Toolset Guide if you want to automate this step with the SchemaExport tool. This tool can create a full SQL DDL, including table definition, custom column type constraints, unique constraints and indexes. If you are using SQL Server, you should also make sure the ASPNET user has permissions to use the database.

We're now ready to start NHibernate's ISession. It is the persistence manager interface, we use it to store and retrieve Cats to and from the database. But first, we've to get an ISession (NHibernate's unit-of-work) from the ISessionFactory:

ISessionFactory sessionFactory =
            new Configuration().Configure().BuildSessionFactory();

An ISessionFactory is responsible for one database and may only use one XML configuration file (Web.config or hibernate.cfg.xml). You can set other properties (and even change the mapping metadata) by accessing the Configuration before you build the ISessionFactory (it is immutable). Where do we create the ISessionFactory and how can we access it in our application?

An ISessionFactory is usually only built once, e.g. at startup inside Application_Start event handler. This also means you should not keep it in an instance variable in your ASP.NET pages, but in some other location. Furthermore, we need some kind of Singleton, so we can access the ISessionFactory easily in application code. The approach shown next solves both problems: configuration and easy access to a ISessionFactory.

We implement a NHibernateHelper helper class:

using System;
using System.Web;
using NHibernate;
using NHibernate.Cfg;

namespace QuickStart
{
    public sealed class NHibernateHelper
    {
        private const string CurrentSessionKey = "nhibernate.current_session";
        private static readonly ISessionFactory sessionFactory;

        static NHibernateHelper()
        {
            sessionFactory = new Configuration().Configure().BuildSessionFactory();
        }

        public static ISession GetCurrentSession()
        {
            HttpContext context = HttpContext.Current;
            ISession currentSession = context.Items[CurrentSessionKey] as ISession;

            if (currentSession == null)
            {
                currentSession = sessionFactory.OpenSession();
                context.Items[CurrentSessionKey] = currentSession;
            }

            return currentSession;
        }

        public static void CloseSession()
        {
            HttpContext context = HttpContext.Current;
            ISession currentSession = context.Items[CurrentSessionKey] as ISession;

            if (currentSession == null)
            {
                // No current session
                return;
            }

            currentSession.Close();
            context.Items.Remove(CurrentSessionKey);
        }

        public static void CloseSessionFactory()
        {
            if (sessionFactory != null)
            {
                sessionFactory.Close();
            }
        }
    }
}

This class does not only take care of the ISessionFactory with its static attribute, but also has code to remember the ISession for the current HTTP request.

An ISessionFactory is threadsafe, many threads can access it concurrently and request ISessions. An ISession is a non-threadsafe object that represents a single unit-of-work with the database. ISessions are opened by an ISessionFactory and are closed when all work is completed:

ISession session = NHibernateHelper.GetCurrentSession();

ITransaction tx = session.BeginTransaction();

Cat princess = new Cat();
princess.Name = "Princess";
princess.Sex = 'F';
princess.Weight = 7.4f;

session.Save(princess);
tx.Commit();

NHibernateHelper.CloseSession();

In an ISession, every database operation occurs inside a transaction that isolates the database operations (even read-only operations). We use NHibernate's ITransaction API to abstract from the underlying transaction strategy (in our case, ADO.NET transactions). Please note that the example above does not handle any exceptions.

Also note that you may call NHibernateHelper.GetCurrentSession(); as many times as you like, you will always get the current ISession of this HTTP request. You have to make sure the ISession is closed after your unit-of-work completes, either in Application_EndRequest event handler in your application class or in a HttpModule before the HTTP response is sent. The nice side effect of the latter is easy lazy initialization: the ISession is still open when the view is rendered, so NHibernate can load unitialized objects while you navigate the graph.

NHibernate has various methods that can be used to retrieve objects from the database. The most flexible way is using the Hibernate Query Language (HQL), which is an easy to learn and powerful object-oriented extension to SQL:

ITransaction tx = session.BeginTransaction();

IQuery query = session.CreateQuery("select c from Cat as c where c.Sex = :sex");
query.SetCharacter("sex", 'F');
foreach (Cat cat in query.Enumerable())
{
    Console.Out.WriteLine("Female Cat: " + cat.Name);
}

tx.Commit();

NHibernate also offers an object-oriented query by criteria API that can be used to formulate type-safe queries. NHibernate of course uses IDbCommands and parameter binding for all SQL communication with the database. You may also use NHibernate's direct SQL query feature or get a plain ADO.NET connection from an ISession in rare cases.

We only scratched the surface of NHibernate in this small tutorial. Please note that we don't include any ASP.NET specific code in our examples. You have to create an ASP.NET page yourself and insert the NHibernate code as you see fit.

Keep in mind that NHibernate, as a data access layer, is tightly integrated into your application. Usually, all other layers depend on the persistence mechanism. Make sure you understand the implications of this design.

A (very) high-level view of the NHibernate architecture:

This diagram shows NHibernate using the database and configuration data to provide persistence services (and persistent objects) to the application.

We would like to show a more detailed view of the runtime architecture. Unfortunately, NHibernate is flexible and supports several approaches. We will show the two extremes. The "lite" architecture has the application provide its own ADO.NET connections and manage its own transactions. This approach uses a minimal subset of NHibernate's APIs:

The "full cream" architecture abstracts the application away from the underlying ADO.NET APIs and lets NHibernate take care of the details.

Heres some definitions of the objects in the diagrams:

Given a "lite" architecture, the application bypasses the ITransaction/ITransactionFactory and/or IConnectionProvider APIs to talk to ADO.NET directly.

An instance of NHibernate.Cfg.Configuration represents an entire set of mappings of an application's .NET types to a SQL database. The Configuration is used to build an (immutable) ISessionFactory. The mappings are compiled from various XML mapping files.

You may obtain a Configuration instance by instantiating it directly. Heres an example of setting up a datastore from mappings defined in two XML configuration files:

Configuration cfg = new Configuration()
    .AddFile("Item.hbm.xml")
    .AddFile("Bid.hbm.xml");

An alternative (sometimes better) way is to let NHibernate load a mapping file from an embedded resource:

Configuration cfg = new Configuration()
    .AddClass(typeof(NHibernate.Auction.Item))
    .AddClass(typeof(NHibernate.Auction.Bid));

Then NHibernate will look for mapping files named NHibernate.Auction.Item.hbm.xml and NHibernate.Auction.Bid.hbm.xml embedded as resources in the assembly that the types are contained in. This approach eliminates any hardcoded filenames.

Another alternative (probably the best) way is to let NHibernate load all of the mapping files contained in an Assembly:

Configuration cfg = new Configuration()
    .AddAssembly( "NHibernate.Auction" );

Then NHibernate will look through the assembly for any resources that end with .hbm.xml. This approach eliminates any hardcoded filenames and ensures the mapping files in the assembly get added.

If a tool like Visual Studio .NET or NAnt is used to build the assembly, then make sure that the .hbm.xml files are compiled into the assembly as Embedded Resources.

A Configuration also specifies various optional properties:

IDictionary props = new Hashtable();
...
Configuration cfg = new Configuration()
    .AddClass(typeof(NHibernate.Auction.Item))
    .AddClass(typeof(NHibernate.Auction.Bind))
    .SetProperties(props);

A Configuration is intended as a configuration-time object, to be discarded once an ISessionFactory is built.

When all mappings have been parsed by the Configuration, the application must obtain a factory for ISession instances. This factory is intended to be shared by all application threads:

ISessionFactory sessions = cfg.BuildSessionFactory();

However, NHibernate does allow your application to instantiate more than one ISessionFactory. This is useful if you are using more than one database.

An ISessionFactory may open an ISession on a user-provided ADO.NET connection. This design choice frees the application to obtain ADO.NET connections wherever it pleases:

IDbConnection conn = myApp.GetOpenConnection();
ISession session = sessions.OpenSession(conn);

// do some data access work

The application must be careful not to open two concurrent ISessions on the same ADO.NET connection!

Alternatively, you can have the ISessionFactory open connections for you. The ISessionFactory must be provided with ADO.NET connection properties in one of the following ways:

If you take this approach, opening an ISession is as simple as:

ISession session = sessions.OpenSession(); // open a new Session
// do some data access work, an ADO.NET connection will be used on demand

All NHibernate property names and semantics are defined on the class NHibernate.Cfg.Environment. We will now describe the most important settings for ADO.NET connection configuration.

NHibernate will obtain (and pool) connections using an ADO.NET data provider if you set the following properties:

This is an example of how to specify the database connection properties inside a web.config:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <configSections>
        <section name="nhibernate" type="System.Configuration.NameValueSectionHandler, System,
                    Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
    </configSections>
    
    <nhibernate>
        <add
            key="hibernate.connection.provider"          
            value="NHibernate.Connection.DriverConnectionProvider" 
        />
        <add 
            key="hibernate.dialect"
            value="NHibernate.Dialect.MsSql2000Dialect"
        />
        <add
            key="hibernate.connection.driver_class"
            value="NHibernate.Driver.SqlClientDriver"
        />
        <add 
            key="hibernate.connection.connection_string" 
            value="Server=127.0.0.1; Initial Catalog=thedatabase; Integrated Security=SSPI" 
        />
        <add 
            key="hibernate.connection.isolation"
            value="ReadCommitted"
        />
        
    </nhibernate>
    
    <!-- other app specific config follows -->
</configuration>

NHibernate relies on the ADO.NET data provider implementation of connection pooling.

You may define your own plugin strategy for obtaining ADO.NET connections by implementing the interface NHibernate.Connection.IConnectionProvider. You may select a custom implementation by setting hibernate.connection.provider_class.

There are a number of other properties that control the behaviour of NHibernate at runtime. All are optional and have reasonable default values.

System-level properties can only be set manually by setting static properties of NHibernate.Cfg.Environment class or be defined in the <nhibernate> section of the application configuration file. These properties cannot be set using Configuration.SetProperties or be defined in the <hibernate-configuration> section of the application configuration file.

hibernate.query.substitutions true=1, false=0

hibernate.query.substitutions toLowercase=LOWER

NHibernate logs various events using Apache log4net.

You may download log4net from http://logging.apache.org/log4net/. To use log4net you will need a log4net configuration section in the application configuration file. An example of the configuration section is distributed with NHibernate in the src/NHibernate.Test project.

We strongly recommend that you familiarize yourself with NHibernate's log messages. A lot of work has been put into making the NHibernate log as detailed as possible, without making it unreadable. It is an essential troubleshooting device. Also don't forget to enable SQL logging as described above (hibernate.show_sql), it is your first step when looking for performance problems.

The interface NHibernate.Cfg.INamingStrategy allows you to specify a "naming standard" for database objects and schema elements.

You may provide rules for automatically generating database identifiers from .NET identifiers or for processing "logical" column and table names given in the mapping file into "physical" table and column names. This feature helps reduce the verbosity of the mapping document, eliminating repetitive noise (TBL_ prefixes, for example). The default strategy used by NHibernate is quite minimal.

You may specify a different strategy by calling Configuration.SetNamingStrategy() before adding mappings:

ISessionFactory sf = new Configuration()
    .SetNamingStrategy(ImprovedNamingStrategy.Instance)
    .AddFile("Item.hbm.xml")
    .AddFile("Bid.hbm.xml")
    .BuildSessionFactory();

NHibernate.Cfg.ImprovedNamingStrategy is a built-in strategy that might be a useful starting point for some applications.

An alternative approach is to specify a full configuration in a file named hibernate.cfg.xml. This file can be used as a replacement for the <nhibernate;> or <hibernate-configuration> sections of the application configuration file.

The XML configuration file is by default expected to be in your application directory. Here is an example:

<?xml version='1.0' encoding='utf-8'?>
<hibernate-configuration xmlns="urn:nhibernate-configuration-2.0">

    <!-- an ISessionFactory instance -->
    <session-factory>

        <!-- properties -->
        <property name="connection.provider">NHibernate.Connection.DriverConnectionProvider</property>
        <property name="connection.driver_class">NHibernate.Driver.SqlClientDriver</property>
        <property name="connection.connection_string">Server=localhost;initial catalog=nhibernate;User Id=;Password=</property>
        <property name="show_sql">false</property>
        <property name="dialect">NHibernate.Dialect.MsSql2000Dialect</property>
        <property name="use_outer_join">true</property>

        <!-- mapping files -->
        <mapping resource="NHibernate.Auction.Item.hbm.xml" assembly="NHibernate.Auction" />
        <mapping resource="NHibernate.Auction.Bid.hbm.xml" assembly="NHibernate.Auction" />

    </session-factory>

</hibernate-configuration>

Configuring NHibernate is then as simple as

ISessionFactory sf = new Configuration().Configure().BuildSessionFactory();

You can pick a different XML configuration file using

ISessionFactory sf = new Configuration()
    .Configure("/path/to/config.cfg.xml")
    .BuildSessionFactory();

Most .NET applications require a persistent class representing felines.

using System;
using Iesi.Collections;

namespace Eg
{
    public class Cat
    {
        private long id; // identifier
        private string name;
        private DateTime birthdate;
        private Cat mate;
        private ISet kittens
        private Color color;
        private char sex;
        private float weight;
    
        public virtual long Id
        {
            get { return id; }
            set { id = value; }
        }
    
        public virtual string Name
        {
            get { return name; }
            set { name = value; }
        }
    
        public virtual Cat Mate
        {
            get { return mate; }
            set { mate = value; }
        }
    
        public virtual DateTime Birthdate
        {
            get { return birthdate; }
            set { birthdate = value; }
        }
    
        public virtual float Weight
        {
            get { return weight; }
            set { weight = value; }
        }
    
        public virtual Color Color
        {
            get { return color; }
            set { color = value; }
        }
    
        public virtual ISet Kittens
        {
            get { return kittens; }
            set { kittens = value; }
        }
    
        // AddKitten not needed by NHibernate
        public virtual void AddKitten(Cat kitten)
        {
            kittens.Add(kitten);
        }
    
        public virtual char Sex
        {
            get { return sex; }
            set { sex = value; }
        }
    }
}

There are four main rules to follow here:

A subclass must also observe the first and second rules. It inherits its identifier property from Cat.

using System;
namespace Eg
{
    public class DomesticCat : Cat
    {
        private string name;

        public virtual string Name
        {
            get { return name; }
            set { name = value; }
        }
    }
}

You have to override the Equals() and GetHashCode() methods if you intend to mix objects of persistent classes (e.g. in an ISet).

This only applies if these objects are loaded in two different ISessions, as NHibernate only guarantees identity ( a == b , the default implementation of Equals()) inside a single ISession!

Even if both objecs a and b are the same database row (they have the same primary key value as their identifier), we can't guarantee that they are the same object instance outside of a particular ISession context.

The most obvious way is to implement Equals()/GetHashCode() by comparing the identifier value of both objects. If the value is the same, both must be the same database row, they are therefore equal (if both are added to an ISet, we will only have one element in the ISet). Unfortunately, we can't use that approach. NHibernate will only assign identifier values to objects that are persistent, a newly created instance will not have any identifier value! We recommend implementing Equals() and GetHashCode() 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):

public class Cat
{

    ...
    public override bool Equals(object other)
    {
        if (this == other) return true;
        
        Cat cat = other as Cat;
        if (cat == null) return false; // null or not a cat

        if (Name != cat.Name) return false;
        if (!Birthday.Equals(cat.Birthday)) return false;

        return true;
    }

    public override int GetHashCode()
    {
        unchecked
        {
            int result;
            result = Name.GetHashCode();
            result = 29 * result + Birthday.GetHashCode();
            return result;
        }
    }

}

Keep in mind that our candidate key (in this case a composite of name and birthday) has to be only valid for a particular comparison operation (maybe even only in a single use case). We don't need the stability criteria we usually apply to a real primary key!

Optionally, a persistent class might implement the interface ILifecycle which provides some callbacks that allow the persistent object to perform necessary initialization/cleanup after save or load and before deletion or update.

The NHibernate IInterceptor offers a less intrusive alternative, however.

public interface ILifecycle
{                                                                    (1)
        LifecycleVeto OnSave(ISession s);                            (2)
        LifecycleVeto OnUpdate(ISession s);                          (3)
        LifecycleVeto OnDelete(ISession s);                          (4)
        void OnLoad(ISession s, object id);
}

OnSave(), OnDelete() and OnUpdate() may be used to cascade saves and deletions of dependent objects. This is an alternative to declaring cascaded operations in the mapping file. OnLoad() may be used to initialize transient properties of the object from its persistent state. It may not be used to load dependent objects since the ISession interface may not be invoked from inside this method. A further intended usage of OnLoad(), OnSave() and OnUpdate() is to store a reference to the current ISession for later use.

Note that OnUpdate() is not called every time the object's persistent state is updated. It is called only when a transient object is passed to ISession.Update().

If OnSave(), OnUpdate() or OnDelete() return LifecycleVeto.Veto, the operation is silently vetoed. If a CallbackException is thrown, the operation is vetoed and the exception is passed back to the application.

Note that OnSave() is called after an identifier is assigned to the object, except when native key generation is used.

If the persistent class needs to check invariants before its state is persisted, it may implement the following interface:

public interface IValidatable
{
        void Validate();
}

The object should throw a ValidationFailure if an invariant was violated. An instance of Validatable should not change its state from inside Validate().

Unlike the callback methods of the ILifecycle interface, Validate() might be called at unpredictable times. The application should not rely upon calls to Validate() for business functionality.

Object/relational mappings are defined in an XML document. The mapping document is designed to be readable and hand-editable. The mapping language is object-centric, meaning that mappings are constructed around persistent class declarations, not table declarations.

Note that, even though many NHibernate users choose to define XML mappings by hand, a number of tools exist to generate the mapping document, including NHibernate.Mapping.Attributes library and various template-based code generators (CodeSmith, MyGeneration).

Let's kick off with an example mapping:

<?xml version="1.0"?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0" assembly="Eg"
    namespace="Eg">

        <class name="Cat" table="CATS" discriminator-value="C">
                <id name="Id" column="uid" type="Int64">
                        <generator class="hilo"/>
                </id>
                <discriminator column="subclass" type="Char"/>
                <property name="BirthDate" type="Date"/>
                <property name="Color" not-null="true"/>
                <property name="Sex" not-null="true" update="false"/>
                <property name="Weight"/>
                <many-to-one name="Mate" column="mate_id"/>
                <set name="Kittens">
                        <key column="mother_id"/>
                        <one-to-many class="Cat"/>
                </set>
                <subclass name="DomesticCat" discriminator-value="D">
                        <property name="Name" type="String"/>
                </subclass>
        </class>

        <class name="Dog">
                <!-- mapping for Dog could go here -->
        </class>

</hibernate-mapping>

We will now discuss the content of the mapping document. We will only describe the document elements and attributes that are used by NHibernate at runtime. The mapping document also contains some extra optional attributes and elements that affect the database schemas exported by the schema export tool. (For example the not-null attribute.)

<hibernate-mapping
         schema="schemaName"                          (1)
         default-cascade="none|save-update"           (2)
         auto-import="true|false"                     (3)
         assembly="Eg"                                (4)
         namespace="Eg"                               (5)
 />

<class
        name="ClassName"                              (1)
        table="tableName"                             (2)
        discriminator-value="discriminator_value"     (3)
        mutable="true|false"                          (4)
        schema="owner"                                (5)
        proxy="ProxyInterface"                        (6)
        dynamic-update="true|false"                   (7)
        dynamic-insert="true|false"                   (8)
        select-before-update="true|false"             (9)
        polymorphism="implicit|explicit"              (10)
        where="arbitrary sql where condition"         (11)
        persister="PersisterClass"                    (12)
        batch-size="N"                                (13)
        optimistic-lock="none|version|dirty|all"      (14)
        lazy="true|false"                             (15)
/>

<id
        name="PropertyName"                      (1)
        type="typename"                          (2)
        column="column_name"                     (3)
        unsaved-value="any|none|null|id_value"   (4)
        access="field|property|nosetter|ClassName(5)">

        <generator class="generatorClass"/>
</id>

<id name="Id" type="Int64" column="uid" unsaved-value="0">
        <generator class="NHibernate.Id.TableHiLoGenerator">
                <param name="table">uid_table</param>
                <param name="column">next_hi_value_column</param>
        </generator>
</id>

<id name="Id" type="Int64" column="cat_id">
        <generator class="hilo">
                <param name="table">hi_value</param>
                <param name="column">next_value</param>
                <param name="max_lo">100</param>
        </generator>
</id>

<id name="Id" type="Int64" column="cat_id">
        <generator class="seqhilo">
                <param name="sequence">hi_value</param>
                <param name="max_lo">100</param>
        </generator>
</id>

<id name="Id" type="String" column="cat_id">
        <generator class="uuid.hex">
            <param name="format">format_value</param>
            <param name="seperator">seperator_value</param>
        </generator>
</id>