hasMany relationship - deselecting set value from view - does not update DB accordingly - grails

Question

I have seen this behaviour on a few different locations lets say:
class fruits {
static hasMany=[preapples:Apples, apples: Apples, postapples: Apples ]
}
static mapping = {
preapples cascade: 'lock'
apples cascade: 'lock'
postapples cascade: 'lock'
}
static constraints = {
preapples nullable:true
apples nullable:true
postapples nullable:true
}
When the views are generated if on fruits I select many apples apple1 apple2 etc
If I return now and change from apple1 + apple2 to just apple1 - everything is ok
If I deselect all as in if there was only 1 selected and I choose ctrl to unhighlight it - which clearly shows nothing has been selected - when I submit form the value is not actually removed.
Is this something to do with nullable ? or am I missing something obvious
I have added nullable true as suggested which has made no difference from selecting values then unselecting it.
I have also modified actual sql table structure (naming to apples and fruits but before making change / dropping table and after making change having it recreate table - there was no difference to nullable (already nullable)
mysql> describe fruits_apples;
+-----------------------------------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------------------------------+------------+------+-----+---------+-------+
| fruits_apples_id | bigint(20) | YES | MUL | NULL | |
| apples_id | bigint(20) | YES | MUL | NULL | |
| fruits_postapples_id | bigint(20) | YES | MUL | NULL | |
| fruits_preapples_id | bigint(20) | YES | MUL | NULL | |
|
mysql> describe fruits_shell_scripts;
+-----------------------------------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------------------------------+------------+------+-----+---------+-------+
| fruits_apples_id | bigint(20) | YES | MUL | NULL | |
| apples_id | bigint(20) | YES | MUL | NULL | |
| fruits_postapples_id | bigint(20) | YES | MUL | NULL | |
| fruits_preapples_id | bigint(20) | YES | MUL | NULL | |
+-----------------------------------+------------+------+-----+---------+-------+
Regards