Domain Driven Design with NHibernate

Ben ScheirmanPrincipal Consultant

Sogeti

www.flux88.com

Start with the Database?

Ok, start with the Model

Domain Driven Design

• Focus on Core Domain first• Work closely with Domain Experts• Learn & Use the “Ubiquitous Language”

Ignore Persistence for Now

Unit Tests

• We’re POCO, baby!

I love NUnit!

Now… what about persistence?

• Pure POCO Model• Ultimate Flexibility

Barriers?

• Granularity• Inheritance• Associations

The problem of Granularity

User

Address

UsersUserIdFirstNameLastNameAddressLine1AddressLine2

CityStateZip

?

The problem of Inheritance

BillingAccount

CreditCard

BillingAccounts

BillingAcountIdcc_number (NULL)exp_date (NULL)TypeRouting_number (NULL)Account_number (NULL)

?CheckingAccount

NHibernate in a Nutshell

• ISessionFactory

• ISession

• ITransaction

HELLO WORLD WITH NHIBERNATEDemo time

Key concepts

• ISessionFactory per application

• ISession per Unit of Work

NHibernate Architecture

ILifecycle

IValidatable

IInterceptor

IUserType

ISession ITransaction IQuery

ISessionFactory

Configuration

Persistent Classes

Domain Model

NHibernate

ADO.NET

Foo.cs.NET

Object

Foos

PK

Column 1

Column 2

Foo.hbm.xmlMapping

How do you GLUE the objects and the database?

XML

Did I just say XML?

Mapping concepts

• <id>

• <property>

• Associations…

Unidirectional One to many

“A Blog has many Posts”

BlogIList<Post> Posts; Post1 *

Blog.hbm.xml

<bag name="Posts"> <key column="BlogId" /> <one-to-many class="Post" /></bag>

BlogsBlogId

BlogName

Author

DateCreated

PostsPostId

BlogId

Title

Body

DatePosted

Bidirectional Many-to-Many

A category has items, an item has categoriesItem

ISet CategoriesCategoryISet Items* *

Items

ItemId

Name

Price

Item_Categories

ItemId

CategoryId

Categories

CategoryId

Name

Item.hbm.xml

<set name="Categories" table="Item_Categories"> <key column="ItemId" /> <many-to-many class="Category" /> </set>

Category.hbm.xml

<set name=“Items" table="Item_Categories“ inverse=“true”> <key column=“CategoryId" /> <many-to-many class=“Item" /> </set>

What is INVERSE?

Understanding INVERSE

Post p = new Post();Category cat = new Category();p.Categories.Add(cat);cat.Posts.Add(p);

INSERT Post_Categories(PostId, CategoryId) VALUES(3, 15)

INSERT Post_Categories(PostId, CategoryId) VALUES(3, 15)

NHibernate needs to “ignore” one of the collections

A DOMAIN DRIVEN DESIGN EXPERIENCE

• A restaurant owner wants us to build him some point-of-sale software for his chain of restaurants.

Can you tell me about how the process works?

Sure, a waiter takes an order for a table. Then he rings it up at

the computer system…

You

Domain Expert

Can you tell me about how the process works?

Sure, a waiter takes an order for a table. Then he rings it up at

the computer system.

You

Domain Expert

Can you elaborate on “rings it up? “ What exactly would he do?

Well, he enters the table number and a ticket opens. Actually, if the there was already a ticket for that

table open, then it would be displayed. Otherwise a new ticket

is created.

We need to record how many guests are at the table when the

ticket is created.

You

Domain Expert

What goes on the ticket?

Oh yeah, the waiter punches in the order for the table. Like what

drinks everyone had, what meal selection. Any menu item really.

Along with the item the price is shown and a running total is

displayed at the bottom.

You

Domain Expert

So the Ticket is like the printed ticket you would receive when

you’re ready to pay the bill?

Yeah, pretty much.

You

Domain Expert

So what happens then?

The waiter will review what he rung up and then send the order.

You

Domain Expert

What happens when the order gets “sent?”

It prints up at the station that prepares the item.

On these “chitters” it has the time the item was ordered, the waiter’s

name, and the table number.

You

Domain Expert

So, each item needs to specify what printers it prints at, correct?

Yes, we should be able to pick from any number of printers, for

example: Bar Printer, Line Printer, Grill Printer, and Dessert Printer.

You

Domain Expert

Say the waiter opens up an existing ticket.

If they ring up additional items and click send, you only want the new

items to get sent, right?

Yeah, that’s correct. I’m thinking that the items that were already

sent would be in a lighter grey font so that it would be easy to see.

You

Domain Expert

So what happens when the guests are ready to pay?

The waiter can take cash or credit payments.

The recorded payments need to satisfy the total before the ticket

can be closed.

You

Domain Expert

Ticket

MenuItem TicketLine

Payment

PrinterStation

CashPayment

CreditPayment

*

*

*

*

*

Resources

• Domain Driven Design (Eric Evans)• ayende.com/blog• Hibernate in Action• NHibernate in Action (pre-release PDFs)• NHibernate Forums• nhusers@googlegroups.com

• Castle Project (Active Record)