Introduction to NHibernate

Introduction to NHibernate By Andrew Smith

The Basics
Object Relation Mapper
Maps POCOs to database tables
Based on Java Hibernate. V stable
Generates SQL at run time
Database agnostic

Entity definitions
“ Persistence ignorance”
No need for special base class
No need to implement special interfaces
Default constructor
Identity property (Primary key)
To support lazy loading, public properties and methods must be virtual
Collection properties must be declared as an interface type

Class Diagram

Configuration
Multiple options – Xml, ActiveRecord, Fluent NH
Xml:
1 overall configuration section
Multiple “<entityname>.hbm.xml” embedded resource files
Intellisense: Place .xsd files in “Microsoft Visual Studio 9.0XmlSchemas”

Sessions
Very quick to create session
Disposable (may hold a DB connection open)
Web: session per request. (Also, ‘long conversations’ - check out NHibernate Burrow)

Mappings: Primary keys
Entities must be identifiable
Multiple strategies for PK
Use ‘native’ as best-guess
Recommended way is “HiLo” to save round-tripping to DB

Mappings: Properties
Maps database columns to properties/fields on entity
Can map columns to nested types
Control insert/update per column
Default is to insert/update all columns, but can be dynamic

Demo: The basics
Fetching single entity by ID
Updating
Inserting

Mappings: Relations
Many types. Most common: bag and set
Assuming bi-directional relations and simple bag collections
In a bi-directional association, only one end can be the ‘owner’
Identify the ‘non-owner’ end of collection by setting Inverse=“true”

Mappings: 1-* Relations
Professor (1) – Class (*)
Bi-directional: <bag> and < many-to-one >
Declare the many-valued end inverse="true“
Use ‘ not-found =ignore’ for bad, legacy data

Mappings: *-* Relations
Student (*) – Class (*)
Bi-directional: 2 <bag> elements with link table name
Arbitrarily choose 1 end to set as Inverse=“true”
Demo

Cascades
Don’t have to explicitly call save / update / delete on related entities
Cascade types: none, save-update, delete, all, all-delete-orphan
Can define a default cascade
Q’s?

Proxies
Enables lazy loading - Just in time loading of data
Lazy loaded collections
Lazy loaded entities (the single-end of a relation)
Controlled via mappings or code
Watch for N+1 issue

Querying
Lots of options:
Criteria
DetachedCriteria
HQL
Linq
SQL

Querying: ICriteria
Out-of-the-box method for building up a query in code
Requires an active session
Weakly typed
nhlambdaextensions project adds typesafe lambda expressions

Querying: Projections
By default NH selects all mapped properties of entity
Projections allow control over the ‘select’ part of generated SQL

Querying: Aliases / Subcriteria
Alias relations to refer to them later
Need to use them to reference multi-level relations. E.g. OrderItem.OrderHeader.Customer.Name

Querying: DetachedCriteria
Same capabilities as ICriteria
Does not require active session
Can be ‘attached’ to any session at any time to execute query

Querying: MultiCriteria
Enables multiple criteria to be evaluated in the one round trip to DB
Very useful for paging

Querying: HQL
Similar to SQL
Allows querying over domain entities
“ HQL is fully object-oriented, understanding notions like inheritance, polymorphism and associations”

Querying: Linq
NHibernate Linq 1.0 released
Supports just about anything you can do with criteria API

Caching: 1 st level cache
Every ISession has an in-built cache called 1 st level cache
Stores all the loaded entities for current unit of work
Prevents needless round-tripping to DB

Caching: 2 nd level cache
Cache shared between sessions
Out of the box support for:
NHibernate.Caches.Prevalence
NHibernate.Caches.SysCache (ASP.Net cache)
NHibernate.Caches.SysCache2 (SQL dependency-based expiration)
NHibernate.Caches.MemCache
Does all the hard work for you!

Querying: Future Queries
Executes a batch of queries in one round trip to DB

Why use it?
Reduces repetitive, error prone data access code
Enables use of OOP
Very flexible
Stable, widely used
Speed up development time
Single place to add behaviour such as auditing / INotifyPropertyChanged / Filters etc
Testing: Easier integration tests
Security: Uses parameterised queries by default

Resources
Book: NHibernate In Action (v 1.2)
NHForge.org
NHibernate blog
Nhibernate FAQ blog
Very active users group (nhusers)
ayende.com
Dimecast and “Summer of NHibernate” videos

Introduction to NHibernate

by Dublin Alt,Net on Sep 07, 2009

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Introduction to NHibernate — Presentation Transcript