How to create new transaction within existing transaction in Grails - grails

Question

I have Machine and Parts domain class
In my service class I am trying to start a transaction, inside that transaction i want to create a new transaction and commit it when it comes out of the inner transaction.
Machine.withTransaction { // transaction 1
// some code to add Parts
// some code to remove Parts
Machine.withNewTrasaction { // transaction 2
// some code to remove Parts.
} // end of transaction 2
// some code to update couple of columns in machine table.
}// end of transaction 1
When it comes out of transaction 2 I want transaction 2 be commited the Parts for the Machine irrespective to transaction 1.But grails is throwing an error back as "Illegal attempt to associate a collection with two open sessions"
How to commit the transaction 2 alone separately without considering the transaction 1?

Answer 1

You can try handling transaction explicitly using #Transactional annotation inside the service class.
Notes:-
Once you add #Transactional annotation to service method, the service class is not deemed transactional by default.
Since you splitting the functionality into two methods, you have to use the proxied instance of the service class to call the second method otherwise you cannot create a new transaction for the second method. Hence, use of applicationContext below in method 1.
You would not need withTransaction or withNewTransaction block any more.
Service class would look like:
class MachineService{
#Transactional
def someMethodToAddParts(){
......
grailsApplication.mainContext.machineService.someMethodToRemoveParts()
......
}
#Transactional(propagation = TransactionDefinition.PROPAGATION_REQUIRES_NEW)
def someMethodToRemoveParts(){
.......
}
}

Answer 2

I've encountered a slightly different problem, maybe it can help somebody, and maybe it could help for the above problem.
I think that the above problem could be avoided by merging entities to the new transaction, then use .merge() method on the collection causing the problem.
First I think the above code looks like this (I've added comments to explain) :
Machine.withTransaction { // transaction 1
// some code to add Parts
yourEntity.addToParts(...)
// some code to remove Parts
Machine.withNewTrasaction { // transaction 2
// some code to remove Parts.
yourEntity.removeFromParts(...)
} // end of transaction 2 -> the session is flushed, and the transaction is committed
// during the flush, hibernate detect that "parts" collection is already attached
// to another session, in another transaction then throw "Illegal
// attempt to associate a collection with two open sessions"
// some code to update couple of columns in machine table.
}
Then the solution is to merge the collection to the new transaction, it give us something like this :
Machine.withTransaction { // transaction 1
// some code to add Parts
yourEntity.addToParts(...)
// some code to remove Parts
Machine.withNewTrasaction { // transaction 2
// some code to remove Parts.
yourEntity.removeFromParts(...)
// Merging the collection to the session
yourEntity.merge() // I haven't tried but maybe you need ensure
// there is a merge cascade on "parts" collection
} // end of transaction 2 -> the session is flushed, and the transaction is committed
// some code to update couple of columns in machine table.
}
In my case, with a merge in the new transaction, I solved the error message "A different object with the same identifier value was already associated with the session : [yourPackage.YourEntity]" (when I talk about YourEntity, you can also read YourDomaineClass)