What’s the difference between session.Merge and session.SaveOrUpdate?

by

This is from section 10.7. Automatic state detection of the Hibernate Reference Documentation:

saveOrUpdate() does the following:

  • if the object is already persistent in this session, do nothing
  • if another object associated with the session has the same identifier,
    throw an exception
  • if the object has no identifier property, save() it
  • if the object’s identifier has the value assigned to a newly
    instantiated object, save() it
  • if the object is versioned (by a <version> or <timestamp>), and the
    version property value is the same value assigned to a newly
    instantiated object, save() it
  • otherwise update() the object

and merge() is very different:

  • if there is a persistent instance with the same identifier currently
    associated with the session, copy the state of the given object onto
    the persistent instance
  • if there is no persistent instance currently associated with the
    session, try to load it from the database, or create a new persistent
    instance
  • the persistent instance is returned
  • the given instance does not become associated with the session, it
    remains detached

You should use Merge() if you are trying to update objects that were at one point detached from the session, especially if there might be persistent instances of those objects currently associated with the session. Otherwise, using SaveOrUpdate() in that case would result in an exception.

More Related Contents:

  1. How to Eager Load Associations without duplication in NHibernate?
  2. Nhibernate: How to represent Many-To-Many relationships with One-to-Many relationships?
  3. What are the First and Second Level caches in (N)Hibernate?
  4. NHibernate ISession Flush: Where and when to use it, and why?
  5. Inverse Attribute in NHibernate
  6. Fighting cartesian product (x-join) when using NHibernate 3.0.0
  7. How do you map an enum as an int value with fluent NHibernate?
  8. JSON.NET and nHibernate Lazy Loading of Collections
  9. List vs Set vs Bag in NHibernate
  10. When to use inverse=false on NHibernate / Hibernate OneToMany relationships?
  11. nHibernate mapping to custom types
  12. IndexOutOfRangeException Deep in the bowels of NHibernate
  13. Is this the right way to eager load child collections in NHibernate
  14. Unable to cast object of type NHibernate.Collection.Generic.PersistentGenericBag to List
  15. How to create OR statements for NHibernate?
  16. Binary Blob truncated to 8000 bytes – SQL Server 2008 / varbinary(max)
  17. How to save a child with assigned id in nhibernate
  18. NHibernate configuration for uni-directional one-to-many relation
  19. Generate XML mappings from fluent Nhibernate
  20. What is the difference between JoinQueryOver and JoinAlias?
  21. How do I view the SQL that is generated by nHibernate?
  22. What should be the lifetime of an NHibernate session?
  23. Get Distinct result set from NHibernate using Criteria API?
  24. DateTime precision in NHibernate and support for DateTime2 in NHibernate SchemeExport
  25. Why doesn’t Entity Framework support ODBC?
  26. NHibernate: difference between Interceptor and Listener
  27. What exactly is “persistence ignorance”?
  28. How do add NOLOCK with nHibernate?
  29. Syntax to define a NHibernate Filter with Fluent Nhibernate?
  30. Do AJAX requests retain PHP Session info?

Leave a Comment