WIP Enhance relation field serialization for image content types

- In better support of the preview image link
- Reuses serialization from existing field serializers
- Flexible adapation allows new types to be added in the future

WIP until preview_image_link has been merged into plone.volto

base.cfg

@@ -18,6 +18,7 @@ parts =
 develop = .
 sources-dir = extras
 auto-checkout =
+    plone.volto
 #    plone.rest
 
 allow-hosts =
@@ -45,6 +46,7 @@ eggs =
     Pillow
     plone.app.debugtoolbar
     plone.restapi [test]
+    plone.volto
 environment-vars =
     zope_i18n_compile_mo_files true
 
@@ -190,6 +192,7 @@ output = ${buildout:directory}/bin/zpretty-run
 mode = 755
 
 [sources]
+plone.volto = git git@github.com:plone/plone.volto.git pushurl=git@github.com:plone/plone.volto.git branch=preview-image-link
 plone.rest = git git://github.com/plone/plone.rest.git pushurl=git@github.com:plone/plone.rest.git branch=master
 plone.schema = git git://github.com/plone/plone.schema.git pushurl=git@github.com:plone/plone.schema.git branch=master
 Products.ZCatalog = git git://github.com/zopefoundation/Products.ZCatalog.git pushurl=git@github.com:zopefoundation/Products.ZCatalog.git

news/xxxx.feature

@@ -0,0 +1 @@
+- Enhance relation field serialization for image content types [@reebalazs]

src/plone/restapi/interfaces.py

@@ -224,3 +224,15 @@ def non_metadata_attributes():
 
     def blocklisted_attributes():
         """Returns a set with attributes blocked during serialization."""
+
+
+class IRelationObjectSerializer(Interface):
+    """The relation object serializer multi adapter serializes the relation object into
+    JSON compatible python data.
+    """
+
+    def __init__(rel_obj, field, context, request):
+        """Adapts relation object, field, context and request."""
+
+    def __call__():
+        """Returns JSON compatible python data."""

src/plone/restapi/serializer/configure.zcml

@@ -113,4 +113,9 @@
       name="plone.restapi.summary_serializer_metadata"
       />
 
+  <!-- Relation object serializer -->
+  <adapter factory=".relationobject.DefaultRelationObjectSerializer" />
+  <adapter factory=".relationobject.ImageRelationObjectSerializer" />
+
+
 </configure>

src/plone/restapi/serializer/relationfield.py

@@ -27,7 +27,25 @@ def relationvalue_converter(value):
 @adapter(IRelationChoice, IDexterityContent, Interface)
 @implementer(IFieldSerializer)
 class RelationChoiceFieldSerializer(DefaultFieldSerializer):
-    pass
+    def __call__(self):
+        result = json_compatible(self.get_value())
+        # Enhance information based on the content type in relation
+        if result is None:
+            return None
+        portal = getMultiAdapter(
+            (self.context, self.request), name="plone_portal_state"
+        ).portal()
+        portal_url = portal.absolute_url()
+        rel_url = result["@id"]
+        if not rel_url.startswith(portal_url):
+            raise RuntimeError(
+                f"Url must start with portal url. [{portal_url} <> {rel_url}]"
+            )
+        rel_path = rel_url[len(portal_url) + 1 :]
+        rel_obj = portal.unrestrictedTraverse(rel_path, None)
+        serializer = getMultiAdapter((rel_obj, self.field, self.context, self.request))
+        result = serializer()
+        return result
 
 
 @adapter(IRelationList, IDexterityContent, Interface)

src/plone/restapi/serializer/relationobject.py

@@ -0,0 +1,81 @@
+from plone.dexterity.interfaces import IDexterityContent
+from plone.restapi.interfaces import IRelationObjectSerializer
+from plone.restapi.serializer.converters import json_compatible
+from zope.component import adapter
+from zope.component import getMultiAdapter
+from zope.interface import implementer
+from zope.interface import Interface
+from zope.interface import alsoProvides
+from plone.app.contenttypes.interfaces import IImage
+from plone.namedfile.interfaces import INamedImageField
+from z3c.relationfield.interfaces import IRelationChoice
+
+import logging
+
+
+log = logging.getLogger(__name__)
+
+
+@adapter(IDexterityContent, IRelationChoice, IDexterityContent, Interface)
+@implementer(IRelationObjectSerializer)
+class DefaultRelationObjectSerializer:
+    def __init__(self, rel_obj, field, context, request):
+        self.context = context
+        self.request = request
+        self.field = field
+        self.rel_obj = rel_obj
+
+    def __call__(self):
+        obj = self.rel_obj
+        # Start from the values of the default field serializer
+        result = json_compatible(self.get_value())
+        # Add some more values from the object in relation
+        additional = {
+            "id": obj.id,
+            "created": obj.created(),
+            "modified": obj.modified(),
+            "UID": obj.UID(),
+        }
+        result.update(additional)
+        return json_compatible(result)
+
+    def get_value(self, default=None):
+        return getattr(self.field.interface(self.context), self.field.__name__, default)
+
+
+class FieldSim:
+    def __init__(self, name, provides):
+        self.__name__ = name
+        alsoProvides(self, provides)
+
+    def get(self, context):
+        return getattr(context, self.__name__)
+
+
+class FieldRelationObjectSerializer(DefaultRelationObjectSerializer):
+    """The relationship object is treatable like a field
+
+    So we can reuse the serialization for that specific field.
+    """
+
+    field_name = None
+    field_interface = None
+
+    def __call__(self):
+        field = FieldSim(self.field_name, self.field_interface)
+        result = super().__call__()
+        # Reuse a serializer from dxfields
+        serializer = getMultiAdapter((field, self.rel_obj, self.request))
+        # Extend the default values with the content specific ones
+        additional = serializer()
+        if additional is not None:
+            result.update(additional)
+        return result
+
+
+@adapter(IImage, IRelationChoice, IDexterityContent, Interface)
+class ImageRelationObjectSerializer(FieldRelationObjectSerializer):
+    # The name of the attribute that contains the data object within the relation object
+    field_name = "image"
+    # The field adapter that we will emulate to get the serialized data for this content type
+    field_interface = INamedImageField