My problem is this: I want to create a grails domain instance, defining the 'Many' instances of another domain that it has. I have the actual source in a Google Code Project but the following should illustrate the problem.

class Person {
  String name
  static hasMany[skills:Skill]

  static constraints = {
   id (visible:false)   
   skills (nullable:false, blank:false)
  }
}

class Skill {
  String name
  String description

  static constraints = {
   id (visible:false)   
   name (nullable:false, blank:false)
   description (nullable:false, blank:false)
  }
}

If you use this model and def scaffold for the two Controllers then you end up with a form like this that doesn't work;

My own attempt to get this to work enumerates the Skills as checkboxes and looks like this;

But when I save the Volunteer the skills are null!

This is the code for my save method;

def save = {
    log.info "Saving: " + params.toString()
    def skills = params.skills
    log.info "Skills: " + skills 
    def volunteerInstance = new Volunteer(params)
    log.info volunteerInstance
    if (volunteerInstance.save(flush: true)) {
        flash.message = "${message(code: 'default.created.message', args: [message(code: 'volunteer.label', default: 'Volunteer'), volunteerInstance.id])}"
        redirect(action: "show", id: volunteerInstance.id)
        log.info volunteerInstance
    }
    else {
        render(view: "create", model: [volunteerInstance: volunteerInstance])
    }
}

This is my log output (I have custom toString() methods);

2010-05-10 21:06:41,494 [http-8080-3] INFO  bumbumtrain.VolunteerController  - Saving: ["skills":["1", "2"], "name":"Ian", "_skills":["", ""], "create":"Create", "action":"save", "controller":"volunteer"]

2010-05-10 21:06:41,495 [http-8080-3] INFO  bumbumtrain.VolunteerController  - Skills: [1, 2]

2010-05-10 21:06:41,508 [http-8080-3] INFO  bumbumtrain.VolunteerController  - Volunteer[ id: null | Name: Ian | Skills [Skill[ id: 1 | Name: Carpenter ] , Skill[ id: 2 | Name: Sound Engineer ] ]] 

Note that in the final log line the right Skills have been picked up and are part of the object instance. When the volunteer is saved the 'Skills' are ignored and not commited to the database despite the in memory version created clearly does have the items. Is it not possible to pass the Skills at construction time? There must be a way round this? I need a single form to allow a person to register but I want to normalise the data so that I can add more skills at a later time.

If you think this should 'just work' then a link to a working example would be great.

If I use the HTML Select then it works fine! Such as the following to make the Create page;

<tr class="prop">
<td valign="top" class="name">
  <label for="skills"><g:message code="volunteer.skills.label" default="Skills" /></label>
</td>
<td valign="top" class="value ${hasErrors(bean: volunteerInstance, field: 'skills', 'errors')}">
    <g:select name="skills" from="${uk.co.bumbumtrain.Skill.list()}" multiple="yes" optionKey="id" size="5" value="${volunteerInstance?.skills}" />
</td>
</tr>   

But I need it to work with checkboxes like this;

<tr class="prop">
<td valign="top" class="name">
  <label for="skills"><g:message code="volunteer.skills.label" default="Skills" /></label>
</td>
<td valign="top" class="value ${hasErrors(bean: volunteerInstance, field: 'skills', 'errors')}">
    <g:each in="${skillInstanceList}" status="i" var="skillInstance">   
      <label for="${skillInstance?.name}"><g:message code="${skillInstance?.name}.label" default="${skillInstance?.name}" /></label>
                                      <g:checkBox name="skills" value="${skillInstance?.id.toString()}"/>
    </g:each>
</td>
</tr> 

The log output is exactly the same! With both style of form the Volunteer instance is created with the Skills correctly referenced in the 'Skills' variable. When saving, the latter fails with a null reference exception as shown at the top of this question.

Hope this makes sense, thanks in advance!

Gav