NHibernate - Relationships


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In this chapter, we will look at relationships in NHibernate. Let's turn our attention to how we can understand relationships in NHibernate. The easiest way is to think about the relationships from the database perspective.

  • We will first create a new application in which we will create some relationships among the customer and order entities.

  • The first relationship we're going to look at is a classic collection relationship.

  • We have a customer with a collection of orders.

  • This is a one-to-many relationship and it's represented in the database by 2 tables and there is a customer ID on the orders table and we have a foreign key relationship back to the customer.

First we need to create a database and two tables Customer and Order. You can create this by specifying the following query in SQL Server Explorer.

USE [master] 
GO 
CREATE DATABASE [NHibernateDemo] 
GO 
USE [NHibernateDemo] 
GO

CREATE TABLE [dbo].[Customer]( 
   [Id] [uniqueidentifier] NOT NULL, 
   [FirstName] [nvarchar](100) NOT NULL, 
   [LastName] [nvarchar](100) NOT NULL, 
   [Points] [int] NULL, [HasGoldStatus] [bit] NULL, 
   [MemberSince] [date] NULL, 
   [CreditRating] [nchar](20) NULL, 
   [AverageRating] [decimal](18, 4) NULL, 
   [Street] [nvarchar](100) NULL, 
   [City] [nvarchar](100) NULL, 
   [Province] [nvarchar](100) NULL, 
   [Country] [nvarchar](100) NULL,
   PRIMARY KEY CLUSTERED ([Id] ASC) 
) 

GO 
CREATE TABLE [dbo].[Order]( 
   [Id] [uniqueidentifier] NOT NULL, 
   [CustomerId] [uniqueidentifier] NULL, 
   [Ordered] [datetime] NULL, 
   [Shipped] [datetime] NULL, 
   [Street] [nvarchar](100) NULL, 
   [City] [nvarchar](100) NULL, 
   [Province] [nvarchar](100) NULL, 
   [Country] [nvarchar](100) NULL, 
   PRIMARY KEY CLUSTERED ([Id] ASC) 
) 
GO

It will create two tables in the database. The following image shows the Customer Table.

Customer Table

The following image shows the Order Table in which you can see the foreign key relationship back to the customer.

Foreign Key Relationship

We need to define the connection string in the app.config file, here is the implementation of the app.config file.

<?xml version = "1.0" encoding = "utf-8" ?> 
<configuration> 
   
   <connectionStrings> 
      <add name = "default" connectionString = "Data Source =
         (localdb)\MSSQLLocalDB;Initial Catalog = NHibernateDemo;Integrated Security =
         True;Connect Timeout = 30;Encrypt = False;TrustServerCertificate = False;
         ApplicationIntent = ReadWrite;MultiSubnetFailover = False"/> 
   </connectionStrings> 

</configuration>

To install the NHibernate in your application, run the following command in NuGet Manager Console window.

install-package NHibernate

To configure the NHibernate configuration, we need to define the configuration in hibernate.cfg.xml file as shown in the following code.

<xml version = "1.0" encoding = "utf-8" ?> 
<hibernate-configuration xmlns = "urn:nhibernate-configuration-2.2"> 
   
   <session-factory> 
      <property name = "connection.connection_string_name">default</property>
		
      <property name = "connection.driver_class">
         NHibernate.Driver.SqlClientDriver 
      </property> 
		
      <property name = "dialect">
         NHibernate.Dialect.MsSql2008Dialect
      </property> 
		
      <property name = "show_sql">true</property> 
   </session-factory> 

</hibernate-configuration>

In this example, we will be working two domain classes, Customer and Order.

Here is the Customer.cs file implementation in which we have two classes, one is the Customer class and another is the Location class in which object is used as an address in the Customer class.

using System; 
using System.Text; 
using Iesi.Collections.Generic;

namespace NHibernateDemo {
 
   public class Customer { 
      
      public Customer() { 
         MemberSince = DateTime.UtcNow; 
         Orders = new HashedSet<Order>(); 
      } 
      
      public virtual Guid Id { get; set; } 
      public virtual string FirstName { get; set; } 
      public virtual string LastName { get; set; } 
      public virtual double AverageRating { get; set; } 
      public virtual int Points { get; set; } 
		
      public virtual bool HasGoldStatus { get; set; }
      public virtual DateTime MemberSince { get; set; } 
      public virtual CustomerCreditRating CreditRating { get; set; } 
      public virtual Location Address { get; set; }
		
      public virtual ISet<Order> Orders { get; set; }
      public virtual void AddOrder(Order order) { Orders.Add(order); order.Customer = this; }
      
      public override string ToString() { 
         var result = new StringBuilder();
			
         result.AppendFormat("{1} {2} ({0})\r\n\tPoints: {3}\r\n\tHasGoldStatus:
            {4}\r\n\tMemberSince: {5} ({7})\r\n\tCreditRating: {6}\r\n\tAverageRating:
            {8}\r\n", Id, FirstName, LastName, Points, HasGoldStatus, MemberSince,
            CreditRating, MemberSince.Kind, AverageRating);
            result.AppendLine("\tOrders:"); 
         
         foreach(var order in Orders) { 
            result.AppendLine("\t\t" + order); 
         } 
			
         return result.ToString(); 
      } 
   }
   
   public class Location { 
      public virtual string Street { get; set; } 
      public virtual string City { get; set; } 
      public virtual string Province { get; set; } 
      public virtual string Country { get; set; } 
   }
   
   public enum CustomerCreditRating { 
      Excellent,
      VeryVeryGood, 
      VeryGood, 
      Good, 
      Neutral, 
      Poor, 
      Terrible 
   } 
}

Here is the mapping file Customer.hbm.xml in which Customer class is mapped to the Customer table.

<?xml version = "1.0" encoding = "utf-8" ?> 
<hibernate-mapping xmlns = "urn:nhibernate-mapping-2.2" assembly = "NHibernateDemo" 
   namespace = "NHibernateDemo"> 
	
   <class name = "Customer"> 
   
      <id name = "Id"> 
         <generator class = "guid.comb"/> 
      </id> 

      <property name = "FirstName"/> 
      <property name = "LastName"/> 
      <property name = "AverageRating"/> 
      <property name = "Points"/> 
      <property name = "HasGoldStatus"/> 
      <property name = "MemberSince" type = "UtcDateTime"/> 
      <property name = "CreditRating" type = "CustomerCreditRatingType"/>
      
      <component name = "Address"> 
         <property name = "Street"/> 
         <property name = "City"/> 
         <property name = "Province"/> 
         <property name = "Country"/> 
      </component> 

   </class> 
</hibernate-mapping>

We also have an Order Class and here is the implementation of Order.cs file.

using System; using Iesi.Collections.Generic;

namespace NHibernateDemo { 

   public class Order { 
      
      public virtual Guid Id { get; set; } 
      public virtual DateTime Ordered { get; set; } 
      public virtual DateTime? Shipped { get; set; }
      public virtual Location ShipTo { get; set; } 
      public virtual Customer Customer { get; set; }
      
      public override string ToString() { 
         return string.Format("Order Id: {0}", Id); 
      } 
   } 
}

Many-to-One Relationship

We also need to map the Order class to the Order table in the database, so here is the implementation of the Order.hbm.xml file.

<?xml version = "1.0" encoding = "utf-8" ?> 
<hibernate-mapping xmlns = "urn:nhibernate-mapping-2.2" assembly = "NHibernateDemo" 
   namespace = "NHibernateDemo"> 

   <class name = "Order" table = "`Order`"> 
   
      <id name = "Id"> 
         <generator class = "guid.comb"/> 
      </id> 
		
      <property name = "Ordered"/> 
      <property name = "Shipped"/> 
   
      <component name = "ShipTo"> 
         <property name = "Street"/> 
         <property name = "City"/> 
         <property name = "Province"/> 
         <property name = "Country"/> 
      </component> 

      <!--<many-to-one name = "Customer" column = "CustomerId" cascade =
         "save-update"/>--> 
			
   </class> 
</hibernate-mapping>

One-to-Many Relationship

Here, we are going to take a look at a one-to-many relationship, in this case, between customer and orders. We've got our customer here, we're creating a new one, and you can see that the collection is initialized with the following pair of orders.

private static Customer CreateCustomer() { 
   
   var customer = new Customer { 
      FirstName = "John", 
      LastName = "Doe", 
      Points = 100, 
      HasGoldStatus = true, 
      MemberSince = new DateTime(2012, 1, 1), 
      CreditRating = CustomerCreditRating.Good, 
      AverageRating = 42.42424242, 
      Address = CreateLocation() 
   }; 
   
   var order1 = new Order { 
      Ordered = DateTime.Now 
   }; 
   
   customer.AddOrder(order1); 
   var order2 = new Order { 
      Ordered = DateTime.Now.AddDays(-1), 
      Shipped = DateTime.Now, 
      ShipTo = CreateLocation() 
   }; 
   
   customer.AddOrder(order2); 
   return customer; 
}

So we will create a new customer and then save it, after saving it, we will find the ID and then reload it in another session in the Main method as shown in the following program.

private static void Main() { 

   var cfg = ConfigureNHibernate(); 
   var sessionFactory = cfg.BuildSessionFactory();
   
   Guid id; 
   using(var session = sessionFactory.OpenSession()) 
	
   using(var tx = session.BeginTransaction()) { 
      var newCustomer = CreateCustomer(); 
      Console.WriteLine("New Customer:"); 
      Console.WriteLine(newCustomer);
      session.Save(newCustomer); 
      id = newCustomer.Id; 
      tx.Commit(); 
   }
   
   using(var session = sessionFactory.OpenSession()) 
	
   using(var tx = session.BeginTransaction()) { 
      var reloaded = session.Load<Customer>(id); 
      Console.WriteLine("Reloaded:"); 
      Console.WriteLine(reloaded); 
      tx.Commit(); 
   }
   
   Console.WriteLine("Press <ENTER> to exit..."); 
   Console.ReadLine(); 
}

Here is the complete Program.cs file implementation.

using System; 
using System.Data; 
using System.Linq; 
using System.Reflection; 

using HibernatingRhinos.Profiler.Appender.NHibernate; 
using NHibernate.Cfg; 
using NHibernate.Dialect; 
using NHibernate.Driver; 
using NHibernate.Linq;

namespace NHibernateDemo {
 
   internal class Program { 
	
      private static void Main() { 
		
         var cfg = ConfigureNHibernate(); 
         var sessionFactory = cfg.BuildSessionFactory();
         
         Guid id; 
         using(var session = sessionFactory.OpenSession())
			
         using(var tx = session.BeginTransaction()) { 
            var newCustomer = CreateCustomer(); 
            Console.WriteLine("New Customer:"); 
            Console.WriteLine(newCustomer); 
            session.Save(newCustomer); 
            id = newCustomer.Id; 
            tx.Commit(); 
         } 
         
         using(var session = sessionFactory.OpenSession()) 
			
         using(var tx = session.BeginTransaction()) { 
            var reloaded = session.Load<Customer>(id); 
            Console.WriteLine("Reloaded:"); 
            Console.WriteLine(reloaded); 
            tx.Commit();
         }
         
         Console.WriteLine("Press <ENTER> to exit..."); 
         Console.ReadLine();
      }
      
      private static Customer CreateCustomer() { 
		
         var customer = new Customer { 
            FirstName = "John", 
            LastName = "Doe", 
            Points = 100, 
            HasGoldStatus = true, 
            MemberSince = new DateTime(2012, 1, 1), 
            CreditRating = CustomerCreditRating.Good, 
            AverageRating = 42.42424242, 
            Address = CreateLocation() 
         }; 
         
         var order1 = new Order { 
            Ordered = DateTime.Now
         }; 
         
         customer.AddOrder(order1); 
			
         var order2 = new Order { 
            Ordered = DateTime.Now.AddDays(-1), 
            Shipped = DateTime.Now, 
            ShipTo = CreateLocation() 
         }; 
         
         customer.AddOrder(order2); 
         return customer; 
      } 
      
      private static Location CreateLocation() { 
		
         return new Location { 
            Street = "123 Somewhere Avenue", 
            City = "Nowhere", 
            Province = "Alberta", 
            Country = "Canada" 
         }; 
      } 
      
      private static Configuration ConfigureNHibernate() { 
         NHibernateProfiler.Initialize(); 
         var cfg = new Configuration(); 
         
         cfg.DataBaseIntegration(x =&ht; { 
            x.ConnectionStringName = "default"; 
            x.Driver<SqlClientDriver>(); 
            x.Dialect<MsSql2008Dialect>(); 
            x.IsolationLevel = IsolationLevel.RepeatableRead; 
            x.Timeout = 10; x.BatchSize = 10; 
         }); 
         
         cfg.SessionFactory().GenerateStatistics();
         cfg.AddAssembly(Assembly.GetExecutingAssembly()); 
         return cfg; 
      } 
   } 
}

When you run this application, you will see the following output.

New Customer:

John Doe (00000000-0000-0000-0000-000000000000)
   Points: 100
   HasGoldStatus: True
   MemberSince: 1/1/2012 12:00:00 AM (Unspecified)
   CreditRating: Good
   AverageRating: 42.42424242

   Orders:
      Order Id: 00000000-0000-0000-0000-000000000000
      Order Id: 00000000-0000-0000-0000-000000000000

Reloaded:
John Doe (9b0fcf10-83f6-4f39-bda5-a5b800ede2ba)
   Points: 100
   HasGoldStatus: True
   MemberSince: 1/1/2012 12:00:00 AM (Utc)
   CreditRating: Good
   AverageRating: 42.4242

   Orders:
Press <ENTER> to exit...

As you can see that initially the customer has 2 orders, but when we reload it, there are no orders to be seen. If you look at customer.hbm.xml file, you can see here that we do not map actual orders collection. So NHibernate knows nothing about it. Let's go ahead and add it.

<?xml version = "1.0" encoding = "utf-8" ?> 
<hibernate-mapping xmlns = "urn:nhibernate-mapping-2.2" 
   assembly = "NHibernateDemo" namespace = "NHibernateDemo">
	
   <class name = "Customer"> 
      
      <id name = "Id">
         <generator class = "guid.comb"/> 
      </id> 
      
      <property name = "FirstName"/>
      <property name = "LastName"/> 
      <property name = "AverageRating"/> 
      <property name = "Points"/> 
      <property name = "HasGoldStatus"/> 
      <property name = "MemberSince" type = "UtcDateTime"/> 
      <property name = "CreditRating" type = "CustomerCreditRatingType"/>
      
      <component name = "Address"> 
         <property name = "Street"/> 
         <property name = "City"/> 
         <property name = "Province"/> 
         <property name = "Country"/> 
      </component>
      
      <set name = "Orders" table = "`Order`"> 
         <key column = "CustomerId"/> 
         <one-to-many class = "Order"/> 
      </set> 
   
   </class> 
</hibernate-mapping>

This is a set and the name of this collection is ‘Orders’, which is stored in a table called order. We need to specify a key which is the name of the foreign key or to find orders. These orders are identified or belong to a customer through the customer ID. And then I have to note that this is a one-to-many relationship and it is with the order class.

We also need to slightly change the Main method by saving the new customer orders to the database as well as shown in the following program.

private static void Main() { 

   var cfg = ConfigureNHibernate(); 
   var sessionFactory = cfg.BuildSessionFactory();
   
   Guid id; 
   using(var session = sessionFactory.OpenSession()) 
	
   using(var tx = session.BeginTransaction()) {
      var newCustomer = CreateCustomer(); 
      Console.WriteLine("New Customer:"); 
      Console.WriteLine(newCustomer);
      session.Save(newCustomer); 
      
      foreach (var order in newCustomer.Orders) { 
         session.Save(order); 
      } 
		
      id = newCustomer.Id; 
      tx.Commit(); 
   }
   
   using(var session = sessionFactory.OpenSession()) 
	
   using(var tx = session.BeginTransaction()) { 
      var reloaded = session.Load<Customer>(id); 
      Console.WriteLine("The orders were ordered by: "); 
      
      foreach (var order in reloaded.Orders) { 
         Console.WriteLine(order.Customer); 
      } 
		
      tx.Commit(); 
   }
   Console.WriteLine("Press <ENTER> to exit..."); Console.ReadLine();
}

We have also specified which customer ordered that particular product. So we need to create a many-to-one relationship to relate that order back to that customer.

So let's go into the Order.hbm.xml file and add a many-to-one, and then name the customer field and the column with the customer ID.

<?xml version = "1.0" encoding = "utf-8" ?> 
<hibernate-mapping xmlns = "urn:nhibernate-mapping-2.2" assembly = "NHibernateDemo"
   namespace = "NHibernateDemo"> 
	
   <class name = "Order" table = "`Order`">
      
      <id name = "Id"> 
         <generator class = "guid.comb"/> 
      </id>
      
      <property name = "Ordered"/> 
      <property name = "Shipped"/> 
      
      <component name = "ShipTo"> 
         <property name = "Street"/> 
         <property name = "City"/> 
         <property name = "Province"/> 
         <property name = "Country"/> 
      </component> 

      <many-to-one name = "Customer" column = "CustomerId"/> 
		
   </class> 
</hibernate-mapping>

Let’s run this application again and now you will see the following output.

New Customer:
John Doe (00000000-0000-0000-0000-000000000000)
   Points: 100
   HasGoldStatus: True
   MemberSince: 1/1/2012 12:00:00 AM (Unspecified)
   CreditRating: Good
   AverageRating: 42.42424242

   Orders:
      Order Id: 00000000-0000-0000-0000-000000000000
      Order Id: 00000000-0000-0000-0000-000000000000

Reloaded:
John Doe (660a6f29-650e-4380-99e0-a5b800febbde)
   Points: 100
   HasGoldStatus: True
   MemberSince: 1/1/2012 12:00:00 AM (Utc)
   CreditRating: Good
   AverageRating: 42.4242

   Orders:
      Order Id: 57314deb-e023-4e55-ac1e-a5b800febbe3
      Order Id: fc065683-d5f5-484b-ae42-a5b800febbe3

The orders were ordered by:
John Doe (660a6f29-650e-4380-99e0-a5b800febbde)
   Points: 100
   HasGoldStatus: True
   MemberSince: 1/1/2012 12:00:00 AM (Utc)
   CreditRating: Good
   AverageRating: 42.4242

   Orders:
      Order Id: 57314deb-e023-4e55-ac1e-a5b800febbe3
      Order Id: fc065683-d5f5-484b-ae42-a5b800febbe3

John Doe (660a6f29-650e-4380-99e0-a5b800febbde)
   Points: 100
   HasGoldStatus: True
   MemberSince: 1/1/2012 12:00:00 AM (Utc)
   CreditRating: Good
   AverageRating: 42.4242

   Orders:
      Order Id: 57314deb-e023-4e55-ac1e-a5b800febbe3
      Order Id: fc065683-d5f5-484b-ae42-a5b800febbe3
		
Press <ENTER> to exit...


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