Transcript Introduction to NHibernate

Introduction to NHibernate
By Andrew Smith
The Basics
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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 9.0\Xml\Schemas”
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: 1st level cache
• Every ISession has an in-built cache
called 1st level cache
• Stores all the loaded entities for current
unit of work
• Prevents needless round-tripping to DB
Caching: 2nd 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?
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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
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Nhibernate FAQ blog
Very active users group (nhusers)
ayende.com
Dimecast and “Summer of NHibernate”
videos