One to one relationship in Silverstripe - silverstripe

Question

I'll try and put this as simply as possible but basically what I am trying to achieve is this.
There are two page types with a one to one relationship, car and owner. I want to be able te be able to select an owner through a dropdown on the car page. If an owner is already linked to another car I don't want it to appear in the dropdown.
I know that I'll need an if statement but I I'm finding it hard to puzzle out how it should go. I followed this tutorial to create the dropdown and it worked quite well.
Thanks in advance.

Answer 1

You can modify the function that gives you the dropdown values. Your DataObject::get() call
can have a filter for the second argument. Simply select all owners that have a CarID of 0.
So, from the tutorial you provided, you can use this modified code:
new DropdownField(
'OwnerID',
'Please choose an owner',
Dataobject::get("Owner","CarID='0'")->map("ID", "Title", "Please Select")
);
2 things to note:
This assumes your DataObjects are called Car and Owner (change as necessary, but keep the ID at the end of the name as it is written above)
This may not work depending how you set up the relationships with the $has_one assignments on your DataObjects. If there is no CarID field on the Owner table, then this code won't help you (you may have it set up vice-versa). In that case, you'll have to create a function that loops through all cars, and then removes the DataObjects from that DataObjectSet that have an OwnerID of 0. Add a comment if this isn't making sense.

Answer 2

Benjamin Smith' answer is perfectly valid for the dropdown you were asking for, just wanted to point to another approach: instead of taking care of the one-to-one relation yourself, there's the 'HasOneComplexTableField' handling this for you.
use the following code for your Car class:
class Car extends Page {
public static $has_one = array(
'Owner' => 'Owner'
);
function getCMSFields() {
$fields = parent::getCMSFields();
$tablefield = new HasOneComplexTableField(
$this,
'Owner',
'Owner',
array(
'Title' => 'Title'
)
);
$tablefield->setParentClass('Car');
$tablefield->setOneToOne();
$tablefield->setPermissions(array());
$fields->addFieldToTab('Root.Content.Owner', $tablefield);
return $fields;
}
}
note the 'setOneToOne()' call, telling the tablefield to only let you select Owners which aren't already selected on another car.
you'll find more information on this in the silverstripe tutorial: http://doc.silverstripe.org/framework/en/tutorials/5-dataobject-relationship-management