How to remember selection with grails

I am new to the grails web framework and coming with intermediate background on JSF. That may be the reason why I needed so much time to find out how to remember selection within tag.

The scenario is following:
When application starts, from the combo box user selects context in which he wants to work and then all queries on domain objects are restricted to this context.

E.g. You select an author, and then when searching for book you always get only books from selected author.

So this selection should be remembered as long as it is not changed. After all, of course, solution is very simple. Just place your selection into session context and then before executing any action read selection from the session.
(This is obvious now but need not to be so obvious to somebody with JSF background as for the most of the time you need not to place explicitly into session when working with JSF)

So let make very short and not full example.

Selection tag may look like this:

g:select name="author.id" from="${Author.list()}" value="${session.authorId}" optionkey="id" noselection="['':'--select author--']"

Action in controller may look like this:

def list = {
if(!params.max) params.max = 50
params.sort = "priority"
params.order = "desc"

def list

//PART WHERE WE TAKE CARE ABOUT AUTHOR BEING CORRECTLY PLACED INTO //SESSION
boolean authorIdInParams = params.author && params.author.id
boolean authorIdInSession = session.authorId

if (!authorIdInSession && !authorIdInParams) {
println "author id unspecified"
[bookList:list]
return
}

// if author is within params it means we have reselected another author
if (authorIdInParams) {
session.authorId = params.author.id
}

// if we are here author is definitly within session
def author = Author.get(session.authorId)

def result = Book.findAllWhere(author:author)

[ authorList: result ]
}