fix: restore required semantics for a bare Link[T]

src/oold/model/_descriptor.py

@@ -189,20 +189,12 @@ def _neutralised(info: Any) -> Any:
             namespace[field_name] = _neutralised(info)
             continue
         if field_name not in namespace and _is_link_annotation(annotation):
-            # `manager: Link[Org]` with nothing assigned. It still needs a field
-            # carrying default=None - a link cannot be required at the pydantic
-            # level, because its value never reaches validation, and left alone
-            # every construction failed with a misleading "Field required" about
-            # a value that had in fact been supplied.
-            #
-            # It is *not* required in the OO-LD sense: that is what
-            # OoldField(required=True) says. Requiredness propagates into
-            # resolution - resolving a link constructs the target - so a
-            # required link makes every stored document lacking it
-            # unconstructible, and a self-referential link could never be
-            # satisfied by a real dataset. Links are declared far more often
-            # than they are required, so the terse form is the common case.
-            namespace[field_name] = OoldField()
+            # `manager: Link[Org]` with nothing assigned. Read as Python reads
+            # it - no default means required - but a link cannot be required as
+            # a pydantic field, because its value never reaches validation.
+            # Left alone, every construction failed with a misleading
+            # "Field required" about a value that had in fact been supplied.
+            namespace[field_name] = OoldField(required=True)
             continue
         # A Field() living in Annotated metadata rather than as the assigned
         # value was never seen here, so its default survived and was evaluated

tests/test_notation.py

@@ -301,22 +301,18 @@ class Old(OoldModel):
     assert Old.__link_fields__["manager"].required_iri is True
 
 
-def test_a_link_annotation_without_a_default_is_optional():
-    """Links are declared far more often than they are required, so the terse
-    form is the common case.
-
-    Requiredness is explicit because it propagates into resolution: resolving a
-    link constructs the target, so a required link makes every stored document
-    lacking it unconstructible - and a self-referential link, `father`, could
-    then never be satisfied by a real dataset.
-    """
+def test_a_link_annotation_without_a_default_is_required():
+    """No default means required, as it does anywhere else in Python. A link is
+    never required at the pydantic level - its value is routed out before
+    validation - so this used to fail with a misleading "Field required" about
+    a value that had in fact been supplied."""
 
     class Bare(OoldModel):
         id: str
         type: str | None = "ex:NBare"
         manager: Link["Org"]
 
     Bare.model_rebuild()
-    assert Bare(id="ex:b").link_iris("manager") is None  # constructs unset
     assert Bare(id="ex:b", manager="ex:acme").link_iris("manager") == "ex:acme"
-    assert Bare.__link_fields__["manager"].required_iri is False
+    with pytest.raises(ValueError, match="manager is required"):
+        Bare(id="ex:b")