Archetypes, AnnotationStorage and image scaling

The ImageField Archetypes schema field type has a methode for scaling the images. But when using AnnotationStorage() this doesn't work out of the box. After defining the sizes for the field:

    atapi.ImageField(  
        'image',  
        storage=atapi.AnnotationStorage(),  
        widget=atapi.ImageWidget(  
            label=_(u"Product Family Image"),  
            description=_(u"add an image to the product family"),  
        ),  
        validators=('isNonEmptyFile'),  
        sizes = {'large'   : (768, 768),  
                 'preview' : (400, 400),  
                 'mini'    : (200, 200),  
                 'thumb'   : (128, 128),  
                },  
    ),  

You need to override bobo_traverse to access the scaled image.

    # workaround to make resized images  
    def __bobo_traverse__(self, REQUEST, name):  
        """Transparent access to image scales  
        """  
        if name.startswith('image'):  
            field = self.getField('image')  
            image = None  
            if name == 'image':  
                image = field.getScale(self)  
            else:  
                scalename = name[len('image_'):]  
                if scalename in field.getAvailableSizes(self):  
                    image = field.getScale(self, scale=scalename)  
            if image is not None and not isinstance(image, basestring):  
                # image might be None or '' for empty images  
                return image  

        return base.ATCTContent.__bobo_traverse__(self, REQUEST, name)  

Now you can access the different sizes of the image by simple urls like "/image_mini"

See also: Products.ATContentTypes.ATNewsItem

Thanks to: http://www.unc.edu/~jj/plone/