httpSession in Grails - grails

Question

I need to access the domain class User of the current session. The following code works:
class RatingController {
def rate = {
def rating = params.rating
def artist = Artist.get( params.id )
def user = User.get(1)
user.addToRatings(new Rating(artist:artist, rating:rating))
user.save()
render(template: "/artist/rate", model: [artist: artist, rating: rating])
}
}
But instead of explicitly get the user with ID equal 1 (User.get(1)) I need to access the user of the current session. I tried the following code, but it doesn't work:
class RatingController {
def rate = {
def rating = params.rating
def artist = Artist.get( params.id )
def user = user.session
user.addToRatings(new Rating(artist:artist, rating:rating))
user.save()
render(template: "/artist/rate", model: [artist: artist, rating: rating])
}
}
I'm still struggling to fully understand the httpSession concept, so a little help would be great.
Thank in advance.
UPDATE
In my UserController, my authentication looks like this:
def authenticate = {
def user = User.findByLoginAndPassword(params.login, params.password)
if(user){
session.user = user
flash.message = "Hello ${user.name}!"
redirect(controller:"event", action:"list")
}else{
flash.message = "Sorry, ${params.login}. Please try again."
redirect(action:"create")
}
}

Answer 1

the http session is nothing more than data that is maintained among a sequence of requests from a single source, like a browser.
To put something on the session, just do
session.setAttribute("key", value)
and to get data off the session just do
session.getAttribute("key")
Grails also adds some fanciness to session access as outlined here. Their example shows
def user = session["user"]
session["user"] = "John"
asset "John" == session.user
Note that if you are using a grails plugin for authentication and authorization, it will probably provide a way to get the user. For example Spring Security gives you the following
springSecurityService.getCurrentUser()
depending on the details of your plugin, that user might or might not be on the session.

Answer 2

In your code I noticed you wrote
def user = user.session
When I think you mean
def user = session.user
The other thing I do is make it session.userId ie session.userId = user.id and then
def user = User.get(session.userId)
Not sure if that's necessary.

Answer 3

You'll need to use .merge():
user.merge(flush: true)
This is because the instance is detached from the persistence context when you save it to HttpSession. Here is the doc from grails:
http://grails.org/doc/2.3.1/ref/Domain%20Classes/merge.html