The NHibernate ISession,
the core of NHibernate usage, has some methods which are quite misunderstood and underused, to name a few, Merge, Persist, Replicate and SaveOrUpdateCopy. Their purpose is: Merge: copies properties from a transient entity to an eventually loaded entity with
the same id in
the first level cache; if there is no loaded entity with
the same id, one will be loaded and placed in
the first level cache first; if using version,
the transient entity must have
the same version as in
the database; Persist: similar to
Save or SaveOrUpdate, attaches a maybe new entity to
the session, but does not generate an INSERT or UPDATE immediately and thus
the entity does not get a database-generated id, it will only get it at flush time; Replicate: copies an instance from one session to another session, perhaps from a different session factory; SaveOrUpdateCopy: attaches a transient entity to
the session and tries to
save it. Here are some samples of its use.
ISession session = ...;
AuthorDetails existingDetails = session.Get<AuthorDetails>(1); //loads an entity and places it in
the first level cache
AuthorDetails detachedDetails = new AuthorDetails { ID = existingDetails.ID, Name = "Changed Name" }; //a detached entity with
the same ID as
the existing one
Object mergedDetails = session.Merge(detachedDetails); //merges
the Name property from
the detached entity into
the existing one;
the detached entity does not get attached
session.Flush(); //saves
the existingDetails entity, since it is now dirty, due to
the change in
the Name property
AuthorDetails details = ...;
ISession session = ...;
session.Persist(details); //details.ID is still 0
session.Flush(); //saves
the details entity now and fetches its id
ISessionFactory factory1 = ...;
ISessionFactory factory2 = ...;
ISession session1 = factory1.OpenSession();
ISession session2 = factory2.OpenSession();
AuthorDetails existingDetails = session1.Get<AuthorDetails>(1); //loads an entity
session2.Replicate(existingDetails, ReplicationMode.Overwrite); //saves it into another session, overwriting any possibly existing one with
the same id; other options are Ignore, where any existing record with
the same id is left untouched, Exception, where an exception is thrown if there is a record with
the same id and LatestVersion, where
the latest version wins
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