Read only the image from the selected cache

Using the instance from the list of cache, the ORM read the image for all the
linked cache which is unnecessary.

Closes #13831
(grafted from 4dd742a6d7cb8765364387d95fbc54c3ed86bce9)

--HG--
branch : 7.0

modules/product_image/product.py

@@ -166,7 +166,8 @@ class Image(ImageMixin, sequence_ordered(), ModelSQL, ModelView, MatchMixin):
             raise ValueError("Invalid size")
         for cache in self.cache:
             if cache.size == size:
-                return cache.image
+                # re-instantiate to fetch only one image
+                return cache.__class__(cache.id).image
         with Transaction().new_transaction():
             cache = self._store_cache(size, self._resize(size))
             # Save cache only if record is already committed

trytond/trytond/ir/avatar.py

@@ -100,7 +100,8 @@ class Avatar(ImageMixin, ResourceMixin, ModelSQL):
             raise ValueError("Invalid size")
         for avatar in self.cache:
             if avatar.size == size:
-                return avatar.image
+                # re-instantiate to fetch only one image
+                return avatar.__class__(avatar.id).image
         if not self.image:
             return None
         if PIL: