[entities] Add inherited_rule method where it may be used from the UI

In later cset, we'll want to retrieve the inherited rule from the management
rule UI. This UI is generated from SEDAArchiveTransfer and
SEDASeqAltArchiveUnitArchiveUnitRefIdManagement entities, so we want this method
on those entity types.

cubicweb_seda/entities/custom.py

@@ -40,6 +40,18 @@ def _climb_rule_holders(transfer_or_archive_unit):
         transfer_or_archive_unit = transfer_or_archive_unit.cw_adapt_to('ITreeBase').parent()
 
 
+def _inherited_rule(self, rule_type):
+    """Return the rule entity of the given type, defined on this unit or in its nearest parent
+    possible defining it, or None if no matching rule has been found.
+    """
+    assert rule_type in RULE_TYPES
+    rtype = 'reverse_seda_{}_rule'.format(rule_type)
+    for rule_holder in _climb_rule_holders(self):
+        if getattr(rule_holder, rtype):
+            return getattr(rule_holder, rtype)[0]
+    return None
+
+
 class SEDAArchiveTransfer(generated.SEDAArchiveTransfer):
 
     def dc_title(self):
@@ -73,6 +85,8 @@ class SEDAArchiveTransfer(generated.SEDAArchiveTransfer):
     def binary_data_objects(self):
         return self.reverse_seda_binary_data_object
 
+    inherited_rule = _inherited_rule
+
 
 class SEDAArchiveUnit(generated.SEDAArchiveUnit):
 
@@ -93,16 +107,7 @@ class SEDAArchiveUnit(generated.SEDAArchiveUnit):
         """
         return self.related('seda_alt_archive_unit_archive_unit_ref_id', 'subject').one()
 
-    def inherited_rule(self, rule_type):
-        """Return the rule entity of the given type, defined on this unit or in its nearest parent
-        possible defining it, or None if no matching rule has been found.
-        """
-        assert rule_type in RULE_TYPES
-        rtype = 'reverse_seda_{}_rule'.format(rule_type)
-        for rule_holder in _climb_rule_holders(self):
-            if getattr(rule_holder, rtype):
-                return getattr(rule_holder, rtype)[0]
-        return None
+    inherited_rule = _inherited_rule
 
 
 class SEDABinaryDataObject(generated.SEDABinaryDataObject):
@@ -217,6 +222,8 @@ class SEDASeqAltArchiveUnitArchiveUnitRefIdManagement(
             return self.reverse_seda_originating_agency_from[0]
         return None
 
+    inherited_rule = _inherited_rule
+
 
 class SEDAKeyword(generated.SEDAKeyword):