Add a hook to Serializer._value_from_field() for complex fields like CompositePrimaryKey

[Tim Graham]

The Python serializer's _value_from_field() method using isinstance for ​special-case handling of CompositePrimaryKey.

Custom model fields can't add similar logic without subclassing the serializer, a cumbersome solution which doesn't scale well in the presence of multiple custom fields.

My motivation for a generic hook is to add serialization support for MongoDB's ​EmbeddedModelField, EmbeddedModelArrayField, PolymorphicEmbeddedModelField, and PolymorphicEmbeddedModelArrayField.

An early prototype:

django/core/serializers/python.py

@@ -39 +39 @@
         data["fields"] = self._current
         return data
 
+    def get_fields_from_model(self, obj, *, polymorphic=False):
+        data = {
+            field.name: self._value_from_field(obj, field)
+            for field in obj._meta.local_fields
+        }
+        if polymorphic:
+            data["_label"] = obj._meta.label
+        return data
+
     def _value_from_field(self, obj, field):
         if isinstance(field, CompositePrimaryKey):
             return [self._value_from_field(obj, f) for f in field]
+        if hasattr(field, "embedded_model"):
+            sub_obj = getattr(obj, field.attname)
+            if sub_obj is None:
+                return None
+            if isinstance(sub_obj, list):
+                return [self.get_fields_from_model(sub) for sub in sub_obj]
+            return self.get_fields_from_model(sub_obj)
+        if hasattr(field, "embedded_models"):
+            sub_obj = getattr(obj, field.attname)
+            if sub_obj is None:
+                return None
+            if isinstance(sub_obj, list):
+                return [
+                    self.get_fields_from_model(sub, polymorphic=True) for sub in sub_obj
+                ]
+            return self.get_fields_from_model(sub_obj, polymorphic=True)
         value = field.value_from_object(obj)
         # Protected types (i.e., primitives like None, numbers, dates,
         # and Decimals) are passed through as is. All other values are

I don't have an immediate proposal for the API change to accomplish this.