CakePHP: If I combine 'fields' and 'contain' in find I don't get the desired result. Can anyone explain why?

I have the following situation: A work has a Image which belongs to a ImageGroup.

If I do this:

$works = $this->Work->find('all', array(
    'contain' => array(
        'PreviewImage' => array(
            'fields' => array('PreviewImage.id', 'PreviewImage.filename', 'PreviewImage.alt'),
            'ImageGroup' => array(
                'fields' => array('ImageGroup.id')
            )
        )
    )
));

I'll get this:

array(
(int) 0 => array(
    'Work' => array(
        'id' => '1',
        'title_us' => 'Work Title',
        'title_de' => 'Arbeit Titel',
        'alias' => 'work-title',
        'pdf_id' => '1',
        'date' => '2013-10-08 10:36:00',
        'title_graphic_us_id' => '1',
        'title_graphic_de_id' => '2',
        'body_us' => 'English Content',
        'body_de' => 'Deutscher Inhalt',
        'preview_image_id' => '3',
        'preview_us' => 'English Preview Text',
        'preview_de' => 'Deutscher Vorschau Text',
        'work_layout_id' => '2',
        'state_id' => '1'
    ),
    'PreviewImage' => array(
        'id' => '3',
        'filename' => 'preview_image.png',
        'alt' => 'Preview Image',
        'image_group_id' => '2',
        'ImageGroup' => array(
            'id' => '2'
        )
    )
)

)

But if I'm trying to reduce the fields for work by the following:

$works = $this->Work->find('all', array(
    'fields' => array(
        'Work.id'
    ),
    'contain' => array(
        'PreviewImage' => array(
            'fields' => array('PreviewImage.id', 'PreviewImage.filename', 'PreviewImage.alt'),
            'ImageGroup' => array(
                'fields' => array('ImageGroup.id')
            )
        )
    )
));

I suddenly don't get the ImageGroup in PreviewImage anymore, see here:

array(
(int) 0 => array(
    'Work' => array(
        'id' => '1'
    ),
    'PreviewImage' => array(
        'id' => '3',
        'filename' => 'preview_image.png',
        'alt' => 'Preview Image',
        'image_group_id' => '2'
    )
)

)

I don't understand why, do you have any idea?

Solution

I'm not 100% sure whether this is the intended behaviour, but you have to include the PreviewImage foreign key field in the fields configuration to get it working:

$works = $this->Work->find('all', array(
    'fields' => array(
        'Work.id',
        'Work.preview_image_id'
    ),
    'contain' => array(
        ...
    )
));

The docs mention to ensure that all necessary foreign keys are included when using the fields option with contain:

When using ‘fields’ and ‘contain’ options - be careful to include all foreign keys that your query directly or indirectly requires.

http://book.cakephp.org/2.0/en/core-libraries/behaviors/containable.html

So this might be something for feature request. Personally I would have expected that this happens automagically.