Hi guys,

ok so I've set up these relationships: "A Museum can have many Exhibitions". I want to link Exhibitions to Museums in the admin gui but I'm a little lost?

Exhibition.php:

class Exhibition extends Page {
...
static $has_one = array(
...
'Museum' => 'Museum'
);

Then in Museum.php:

class Museum extends Page {
...
static $has_many = array(
   'Exhibitions' => 'Exhibition'
);
}

... any ideas what magic I have to do to change getCMSFields so that under an Exhibition I get a dropdown to choose which Museum it belongs to, and under a Museum it lists / enables changing of Exhibitions?

Yes I could make Exhibitions children of Museums but there are reasons why I don't want to do this, and besides, it wouldn't work for many_many -- and so I have a similar question for that..

It looks like RelatedDataEditor is the way to do this but when I tried this it crashed because it enforced 1..* not 0..* ?

thanks very much!

Julian (Kiwi in Wimbledon)

function getCMSFields() {
$fields = parent::getCMSFields();
$fields->addFieldToTab('Root.Main',new DropdownField('MuseumID','Museum'));
return $fields;
}

Thats the simplest solution for a has_one. If your relation is SiteTree-based (rather than extending DataObject), have a look at TreeDropdownField (or TreeMultiSelectField for has_many).
TableListField, TableField and ComplexTableField basically do the same thing as RelatedDataEditor for has_many-Relationsships. They are very versatile ways to display relationship, unfortunately quite poorly documented ;)

We've got a very brief documentation on http://doc.silverstripe.com/doku.php?id=datamodel.

We're using Object Relational Mapping (http://en.wikipedia.org/wiki/Object-relational_mapping) and basically the same conventions as Ruby on Rails to build automatic database-relations. Hence, when you write something like
static $has_one = array("Museum"=>"Museum");
our db-builder adds a MuseumID-column to the table.

I forgot an important chunk of code that possibly confused you:
function getCMSFields() {
   $fields = parent::getCMSFields();
   $fields->addFieldToTab(
      'Root.Main',
      new DropdownField(
         'MuseumID',
         'Museum',
         DataObject::get('Museum')->toDropDownMap()
      )
   );
   return $fields;
}
This adds an actual value-map to the dropdown (see DataObjectSet for more info on this).