class User {
    String name
    static hasMany = [files : File]
}

class File {
    long size
}

def user = User.get(id)
user.files.each(total+=it.size)

def c = File.createCriteria()
def f = c.list{
    eq("user",  user)   // What here ?
    projections
        {
        sum("size")
        }
    }

User.executeQuery(
   'select sum(file.length) from User u join u.files file where u.id=:userId',
   [userId: id])[0]

def user = User.get(id)
def sum = File.executeQuery("select sum(f.size) from File f where f.user = ?", user);

def total = user.files.sum {it.size}