Fix single-valued UID reference fields not settable through the JSON API

The UIDReferenceFieldMixin always passed a list to the field setter, even
for single-valued fields. AT UIDReferenceField validators reject a list
value ('[...] is not supported'), so assigning e.g. a Sample's
Specification, Batch or SamplePoint through the API failed. Unwrap the
list to a scalar (or None to clear) for non multi-valued fields, matching
the classic ReferenceFieldManager.

Author: ramonski

docs/changelog.rst

@@ -15,6 +15,7 @@ Changelog
    applied. Uninstalling from the same panel removes the PAS plugin
    and the per-user JWT signing secrets.
 
+- #106 Fix single-valued UID reference fields not settable through the JSON API
 - #94 Extract registry and settings helpers to api/settings
 - #93 Convert api module into a package and extract user helpers
 - #92 Restrict /registry, /settings, /users to prevent info leaks

src/senaite/jsonapi/fieldmanagers.py

@@ -664,14 +664,18 @@ def set(self, instance, value, **kw):  # noqa
             elif api.is_path(v):
                 refs.append(api.get_object_by_path(v))
 
-        # Handle non multi valued fields
+        # convert all references to UIDs
+        refs = [str(api.get_uid(ref)) for ref in refs if ref]
+
+        # Single valued fields expect a scalar value, not a list. Passing a
+        # list makes the field validator of e.g. an AT UIDReferenceField
+        # reject the value with "[...] is not supported", so unwrap it here
+        # (None clears the reference). Multi valued fields keep the list.
         if not self.multi_valued:
             if len(refs) > 1:
                 raise ValueError("Multiple values given for single valued "
                                  "field {}".format(repr(self.field)))
-
-        # convert all references to UIDs
-        refs = [str(api.get_uid(ref)) for ref in refs if ref]
+            refs = refs[0] if refs else None
 
         return self._set(instance, refs, **kw)