OK, I have the following dataobject:

<?php

class TestObject extends DataObject {

	static $db = array(
	);

	static $has_one = array(
	'FileName' => 'Image'
	);

	static $belongs_many_many = array(
		'TestPages' => 'TestPage'
	);

	function getCMSFields_forPopup() {
		$fields = new FieldSet();
		$fields->push( new ImageField( 'FileName', 'ColorImage' ) );
		return $fields;
	}

}
?>

And then I have the following code in TestPage.php:

	static $many_many = array(
		'TestObjects' => 'TestObject'
	);
	
	function getCMSFields() {
		$fields = parent::getCMSFields();
		$testTablefield = new ManyManyComplexTableField(
			$this,
			'TestObjects',
			'TestObject',
			array(
				'FileNameID' => 'File'
			),
			'getCMSFields_forPopup'
		);
		$testTablefield->setPageSize(100);
		$testTablefield->setAddTitle( 'a TestObject' );
		$fields->addFieldToTab( 'Root.Content.TestObjects', $testTablefield );
		return $fields;
	}

Everything works, saves, etc. But: in the many-many table in the TestObjects tab, the FileName shows up as an ID. If I remove the "ID" from FileName, I get a server error. How can I show the name of the file, or the url, in the many-many table?

Thanks!

ok thats because the field on your TestObject isnt 'FileName' as you defined in your has_one. Rather it creates a seperate table called FileName and then a column in your testobject called 'FileNameID' so from your TestObject the only field on FileName it knows about is is FileNameID.

Now the most straight forward way to get this to work is to define a method on TestObject which returns the File as a dataobject. So on TestObject you can add something like

function getImageFileName() {
if($this->FileNameID) {
$file = DataObject::get_by_id("Image", $this->FileNameID); // returns the file object
return ($file) ? $file->Filename : "No File Name";
}
return "No File Name";
}

then in your table field you can call ImagesFileName ?.

 $testTablefield = new ManyManyComplexTableField( 
         $this, 
         'TestObjects', 
         'TestObject', 
         array( 
            'ImagesFileName' => 'File Name' 
         ), 
         'getCMSFields_forPopup' 
      );

Thanks willr for your answer, I will try. I'm kind of swamped right now, but I'll report back, I promise!