fix: support Pydantic MISSING sentinel in ops.Relation.save

ops/model.py

@@ -1873,6 +1873,10 @@ def _add_transfer(self):
                 # data.destination will be stored under the Juju relation key 'to'
                 relation.save(data, self.unit)
 
+        If a Pydantic model's ``model_dump`` method omits any field (e.g. if its
+        value is Pydantic's ``MISSING`` sentinel) the field will be erased from
+        the relation data.
+
         Args:
             obj: an object with attributes to save to the relation data, typically
                 a Pydantic ``BaseModel`` subclass or dataclass.
@@ -1915,7 +1919,11 @@ def _add_transfer(self):
             values = {field: getattr(obj, field) for field in fields}
 
         # Encode each value, and then pass it over to Juju.
-        data = {field: encoder(values[attr]) for attr, field in sorted(fields.items())}
+        # Missing values are erased from the databag using empty string values.
+        data = {
+            field: encoder(values[attr]) if attr in values else ''
+            for attr, field in sorted(fields.items())
+        }
         self.data[dst].update(data)
 
 

test/test_model_relation_data_class.py

@@ -31,6 +31,11 @@
 except ImportError:
     pydantic = None
 
+try:
+    from pydantic.experimental.missing_sentinel import MISSING
+except ImportError:
+    MISSING = None  # type: ignore
+
 import ops
 from ops import testing
 
@@ -213,6 +218,34 @@ def databag_class(self) -> type[DatabagProtocol]:
     _test_classes.extend((MyPydanticDataclassCharm, MyPydanticBaseModelCharm))
 
 
+if pydantic and MISSING is not None:
+
+    class MissingPydanticDatabag(pydantic.BaseModel):
+        foo: str
+        bar: int = pydantic.Field(default=0, ge=0)
+        baz: list[str] = pydantic.Field(default_factory=list)
+        quux: Nested = pydantic.Field(default_factory=Nested)
+        miss: str | MISSING = MISSING  # type: ignore
+
+        @pydantic.field_validator('baz')
+        @classmethod
+        def check_foo_not_in_baz(cls, baz: list[str], values: Any):
+            data = cast('dict[str, Any]', values.data)
+            foo = data.get('foo')
+            if foo in baz:
+                raise ValueError('foo cannot be in baz')
+            return baz
+
+        model_config = pydantic.ConfigDict(validate_assignment=True)
+
+    class MissingPydanticBaseModelCharm(BaseTestCharm):
+        @property
+        def databag_class(self) -> type[DatabagProtocol]:
+            return MissingPydanticDatabag
+
+    _test_classes.append(MissingPydanticBaseModelCharm)
+
+
 @pytest.mark.parametrize('charm_class', _test_classes)
 def test_relation_load_simple(charm_class: type[BaseTestCharm]):
     class Charm(charm_class):