Introduction to NHibernate
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NHibernate is an object-relational mapper that maps plain old CLR objects (POCOs) to database tables. It is based on Hibernate and provides a stable, database agnostic way to generate SQL at runtime. NHibernate configurations can be defined using XML, attributes or code-first fluent APIs. Sessions represent a lightweight transaction and identity mapping unit of work with the database. Entities are mapped to tables through properties, identifiers and relationships such as one-to-many, many-to-many and inheritance. Queries can be performed using the criteria API, HQL, LINQ or future queries to optimize performance. Caching at the session and query levels improves efficiency. NHibernate reduces
Introduction to NHibernate
- 1. Introduction to NHibernate By Andrew Smith
- 2. The Basics Object Relation Mapper Maps POCOs to database tables Based on Java Hibernate. V stable Generates SQL at run time Database agnostic
- 3. 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
- 5. 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”
- 6. Sessions Very quick to create session Disposable (may hold a DB connection open) Web: session per request. (Also, ‘long conversations’ - check out NHibernate Burrow)
- 7. 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
- 8. 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
- 9. Demo: The basics Fetching single entity by ID Updating Inserting
- 10. 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”
- 11. 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
- 12. Mappings: *-* Relations Student (*) – Class (*) Bi-directional: 2 <bag> elements with link table name Arbitrarily choose 1 end to set as Inverse=“true” Demo
- 13. 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?
- 14. 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
- 15. Querying Lots of options: Criteria DetachedCriteria HQL Linq SQL
- 16. 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
- 17. Querying: Projections By default NH selects all mapped properties of entity Projections allow control over the ‘select’ part of generated SQL
- 18. 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
- 19. Querying: DetachedCriteria Same capabilities as ICriteria Does not require active session Can be ‘attached’ to any session at any time to execute query
- 20. Querying: MultiCriteria Enables multiple criteria to be evaluated in the one round trip to DB Very useful for paging
- 21. Querying: HQL Similar to SQL Allows querying over domain entities “ HQL is fully object-oriented, understanding notions like inheritance, polymorphism and associations”
- 22. Querying: Linq NHibernate Linq 1.0 released Supports just about anything you can do with criteria API
- 23. 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
- 24. 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!
- 25. Querying: Future Queries Executes a batch of queries in one round trip to DB
- 26. 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
- 27. 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