[#1384] Add custom to_native method to Base64ImageField

Add Base64ImageField.to_native() to include the full path to the image
in any API use and add the option of requesting a thumbnail as part of an
API get request.

See docstring and comments in to_native() for more details.

akvo/rest/fields.py

@@ -7,7 +7,6 @@
 
 import base64
 import imghdr
-import os
 import six
 import uuid
 
@@ -16,8 +15,7 @@
 
 from rest_framework import serializers
 from rest_framework.fields import ImageField
-
-from akvo import settings
+from sorl.thumbnail import get_thumbnail
 
 
 class NonNullCharField(serializers.CharField):
@@ -69,10 +67,49 @@ def from_native(self, base64_data):
         return super(Base64ImageField, self).from_native(data)
 
     def to_native(self, value):
-        """ Return the full path to the image, including the MEDIA_URL part
         """
-        value = super(Base64ImageField, self).to_native(value)
-        return os.path.join(settings.MEDIA_URL, value)
+        :param value: A Base64ImageField object
+        :return: The full path to the image. If the request includes a special query string param a thumbnail is
+            generated and returned along with the original
+
+        If the querystring part of the request includes the query param "image_thumb_name" a thumbnail is generated,
+        provided that at least one of "width" and "height" parameters is also supplied. The image_thumb_name value is
+        used as the key for the thumbnail in the dict returned.
+        If a thumbnail is included a dict with two members is returned:
+        {
+            'original': '/full/path/to/original/image.png',
+            '<value of image_thumb_name QS param>': '/full/path/to/thumbnail/image.png'
+        }
+        This dict will be converted as appropriate to JSON or XML
+
+        NOTE: This special functionality works best when there is only one image field in a model. If there are more,
+        things will still work (I think), but all thumbs returned will have the same dimensions
+        """
+        ORIGINAL_KEY = u'original'
+        if value:
+            # get the full path of original image
+            image_url = value.url
+            image_thumb_name = self.context['request'].GET.get('image_thumb_name')
+            # do we have a request for a thumbnail?
+            # also make sure the thumb isn't named "original"!
+            if image_thumb_name and image_thumb_name.lower() != ORIGINAL_KEY:
+                width = self.context['request'].GET.get('width')
+                height = self.context['request'].GET.get('height')
+                # set thumb size according to what data we got
+                if width and height:
+                    thumb = get_thumbnail(value, '{}x{}'.format(width, height), quality=99)
+                elif width:
+                    thumb = get_thumbnail(value, '{}'.format(width), quality=99)
+                elif height:
+                    thumb = get_thumbnail(value, 'x{}'.format(height), quality=99)
+                else:
+                    # bail if we get neither width nor heigth
+                    return image_url
+                # populate dict with original and thumb paths
+                image_dict = {ORIGINAL_KEY: image_url}
+                image_dict[image_thumb_name] = thumb.url
+                return image_dict
+            return image_url
 
     def get_file_extension(self, filename, decoded_file):
         extension = imghdr.what(filename, decoded_file)