NHibernate - Cascades


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In this chapter, we will be covering how to use the Cascade feature. If you have a set or a collection of items or a relationship between two classes such as our customer and order and have a foreign key relationship. If we delete the customer by default, NHibernate doesn't do anything to the child objects, so the ones that belong to that customer and we could be orphaning orders.

  • We could also be violating foreign key constraints, so we can use the notion of cascades.

  • By default, NHibernate does not cascade operations to child objects.

  • The reason for this is that you can have relationships such as a customer having a default shipping address and that shipping address is shared with many different customers.

  • So you wouldn't want to cascade that relationship necessarily because other customers are still referring to it.

  • So the whole notion of cascades is to tell NHibernate how to handle its child entities.

There are different options for cascading, which are as follows −

  • none − which is the default and it means no cascading.

  • all − which is going to cascade saves, updates, and deletes.

  • save-update − it will cascade, saves and updates.

  • delete − it will cascade deletes.

  • all-delete-orphan − it is a special one which is quite frequently used and is the same as All Except, if it finds Delete-orphan rows, it will delete those as well.

You can specify the default in your hbm.xml file, so you can provide a default cascade on that Hibernate mapping element or you can also specify it for specific collections and relationships such as the many-to-one.

Let’s have a look into simple example cascades, let's fix the problem in the program, where we have to manually cascade the save to the orders as shown in the following code.

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(); 
}

In the above code snippet, you can see that we are manually saving all the orders for the customer. Now let’s remove manual cascade code in which all the orders are saved.

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(); 
}

We need to specify the cascade option in customer.hbm.xml.

<?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`" cascade = "all-delete-orphan"> 
         <key column = "CustomerId"/> 
         <one-to-many class = "Order"/> 
      </set> 
   
   </class> 
</hibernate-mapping>
  • Now, orders fully belong to the customer. So if the customers were deleted from the database, our application here would want to delete all of those orders, including any that might have been orphaned.

  • It will end up doing a delete. By that, it will say delete from order table, where the customer ID equals the customer that you're deleting.

  • So you can actually cascade these deletes. So with the All, it will do saves, updates, and deletes.

Now 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 (10b2a3d7-7fcf-483c-b1da-a5bb00b8512e)
   Points: 100
   HasGoldStatus: True
   MemberSince: 1/1/2012 12:00:00 AM (Utc)
   CreditRating: Good
   AverageRating: 42.4242

   Orders:
      Order Id: e6680e30-5b3b-4efa-b017-a5bb00b85133
      Order Id: b03858e7-8c36-4555-8878-a5bb00b85134

The orders were ordered by:
John Doe (10b2a3d7-7fcf-483c-b1da-a5bb00b8512e)
   Points: 100
   HasGoldStatus: True
   MemberSince: 1/1/2012 12:00:00 AM (Utc)
   CreditRating: Good
   AverageRating: 42.4242

   Orders:
      Order Id: e6680e30-5b3b-4efa-b017-a5bb00b85133
      Order Id: b03858e7-8c36-4555-8878-a5bb00b85134

John Doe (10b2a3d7-7fcf-483c-b1da-a5bb00b8512e)
   Points: 100
   HasGoldStatus: True
   MemberSince: 1/1/2012 12:00:00 AM (Utc)
   CreditRating: Good
   AverageRating: 42.4242

   Orders:
      Order Id: e6680e30-5b3b-4efa-b017-a5bb00b85133
      Order Id: b03858e7-8c36-4555-8878-a5bb00b85134
		
Press <ENTER> to exit...

As you can see that we have deleted the code from the program that manually cascaded and our application is still working.

So depending on your relationship, you might want to cascade those. Now, let's take a look at a different cascade relationship. Let’s go to the Order.hbm.xml file and we can cascade that many-to-one relationship.

<?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>

So if we create a new order and there's a new customer attached to it and we say, save that order, we might want to cascade that. But one thing that we'd probably don't want to do is if an order is deleted to delete the corresponding customer.

So here, we would want to do a save update, so using a save-update, it will cascade any saves or updates to that customer. So, if we get a new customer or if we are changing the customer, it will cascade that. If it is a delete, it won't delete that from the database.

So running our application again, everything still works as expected.

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

Reloaded:
John Doe (10b2a3d7-7fcf-483c-b1da-a5bb00b8512e)
   Points: 100
   HasGoldStatus: True
   MemberSince: 1/1/2012 12:00:00 AM (Utc)
   CreditRating: Good
   AverageRating: 42.4242

   Orders:
      Order Id: e6680e30-5b3b-4efa-b017-a5bb00b85133
      Order Id: b03858e7-8c36-4555-8878-a5bb00b85134

The orders were ordered by:
John Doe (10b2a3d7-7fcf-483c-b1da-a5bb00b8512e)
   Points: 100
   HasGoldStatus: True
   MemberSince: 1/1/2012 12:00:00 AM (Utc)
   CreditRating: Good
   AverageRating: 42.4242

   Orders:
      Order Id: e6680e30-5b3b-4efa-b017-a5bb00b85133
      Order Id: b03858e7-8c36-4555-8878-a5bb00b85134
      John Doe (10b2a3d7-7fcf-483c-b1da-a5bb00b8512e)
		
   Points: 100
   HasGoldStatus: True
   MemberSince: 1/1/2012 12:00:00 AM (Utc)
   CreditRating: Good
   AverageRating: 42.4242

   Orders:
      Order Id: e6680e30-5b3b-4efa-b017-a5bb00b85133
      Order Id: b03858e7-8c36-4555-8878-a5bb00b85134
		
Press <ENTER> to exit...

Now you should have a look at your application, remember that the default is None and you have to think about your entities and your relationships between them to determine the appropriate cascades for each of your entities as well as each of your relationships in that database.



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