(Quick Reference)
merge
Purpose
Merges a domain class instance back into the current persistence context and returns a new merged instance.
Examples
def b = new Book(title: "The Shining")
b = b.merge()
Description
The
merge method is similar in function to the
save method, but not in behaviour.
merge allows the merging of "detached" instances such as those stored in the HTTP
session. Each persistent instance is associated with a persistence context. A new persistence context is created for each
request. The result is that objects stored in the HTTP session lose their persistence context on subsequent requests. In this case you can't simply call
save as the domain class is not associated with a current context.
The
merge method on the other hand lets you merge a detached object's state back into the current Hibernate session. Unlike the
save method this method returns a new instance of the class representing the re-attached object. In other words you
must write code like this:
If you don't use the return value of the
merge method then you still have access to the original unmodified detached instance and you will get errors such as lazy initialization exceptions.
The
merge method is defined in the Hibernate documentation as follows:
Copy the state of the given object onto the persistent object with the same identifier. If there is no persistent instance currently associated with the session, it will be loaded. If the given instance is unsaved, save a copy of and return it as a newly persistent instance.
The
merge method is equivalent to the Hibernate
merge method.
Parameters:
validate (optional) - Set to false if validation should be skippedflush (optional) - When set to true flushes the persistence context, hence persisting the object immediately
