Associating With NHibernate Object Relational Mapper For 'Net

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Associating With NHibernateObject Relational
MapperFor .Net

• Oren Eini
• Orene_at_we-can.co.il

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Associations - One To Many
blog.Posts

• ltset name'Posts'
• access'NHibernate.Generics.GenericAccessorCamel
  Case, NHibernate.Generics'
• lazy'true' cascade'all-delete-orphan'
  inverse'true'gt
• ltkey column'post_blogid' /gt
• ltone-to-many class'NHibernate.Generics.Tests.Pos
  t, NHibernate.Generics.Tests' /gt
• lt/setgt

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Associations Many To One
post.Blog

• ltmany-to-one name'Blog' column'post_blogid'
  access'NHibernate.Generics.GenericAccessor,
  NHibernate.Genericsclass'NHibernate.Generics.Te
  sts.Blog, NHibernate.Generics.Tests' /gt

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Associations Many to Many

• ltset name'Users' access'NHibernate.Generics.Gene
  ricAccessor, NHibernate.Generics'
• table'UsersBlogs' lazy'true'
  cascade'save-update' inverse'true'gt
• ltkey column'blog_id' /gt
• ltmany-to-many column'user_id'
• class'NHibernate.Generics.Tests.User,
  NHibernate.Generics.Tests'/gt
• lt/setgt

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Two way associations

• Maintaining Integrity Of Associations
• blog.Posts.Add(post)
• post.Blog blog
• Smart Collections (NHibernate.Generics)
• posts new EntitySetltPostgt(delegate(Post p)
  p.Blog this ,delegate(Post p) p.Blog
  null )
• blog new EntityRefltBloggt(delegate(Tests.Blog
  b) b.Posts.Add(this) ,delegate(Tests.Blog b)
  b.Posts.Remove(this))

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NHibernate Collections

• Set (ISet)
• Most common collection for database.
• No duplicates
• No indexer
• List (IList)
• Required some sort of indexer in database
• Meaningful Order
• Bag (IList)
• Allows duplicates
• Has indexer
• Not recommended in general

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NHibernate Collections - Continued

• Map (IDictionary)
• Key / Value Pair
• Can use entities / value types / components as
  keys / values
• All Collections can be ordered / unordered
• Recommended Make collections lazy to avoid
  loading too much, too often.
• Advanced
• Collections could be filtered
• Custom Collections

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Collections Lazy Loading

• Collections are non-lazy by default.
• Without Lazy Load - Problem All The Database Is
  In Memory
• When you load an object, all its collections are
  loaded, and all their references are loaded, and
  all their collections, etc.
• With Lazy Load - Problem Select N 1
• Load a set object, then access their collections,
  issue a select per object in the set.
• Both problems can be mitigated by selective use
  of lazy loading and eager fetching.

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Collection Ownership

• Owning the collection - one of the sides of the
  collection is responsible to save the
  association to the database.
• When the association is one to many, the
  responsibility is usually on the many side, since
  the value is on its table.
• When the association in many to many, one of the
  sides need to be chosen. This is a policy
  decision. Saving the other side after adding an
  association will not update the database.

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Cascades

• NHibernate cascades operate similarly to database
  FK constraints, and serve the same purpose.
• Cascade Options
• None
• Save-Update
• All
• Delete
• Delete-Orphans
• All-Delete-Orphans
• Note Correct cascades are crucial for the
  application.

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A Few Words Of Advise

• Doing blog.Posts.Count causes NHibernate to
  load the whole collection to memory (if lazy).
  Wasteful in resources. Use HQL for this.
• Eager fetch if you need, try to limit use of ad
  hoc lazy loading if possible.
• The session that loaded the object is tied to its
  lazy load behavior, if the session is closed and
  a lazy loaded collection is accessed, an
  exception is thrown.
• Think carefully about lazy loading and cascades,
  changes in those can cause cascading affects to
  the application (pun intended).

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