Implement flexible entity relation system (Phases A–E)

Declarative relation registry via defrelation s-expressions with
cardinality enforcement (one-to-one, one-to-many, many-to-many),
registry-aware relate/unrelate/can-relate API endpoints, generic
container-nav fragment, and relation-driven UI components.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

relations/bp/data/routes.py

@@ -35,22 +35,43 @@ def register() -> Blueprint:
         parent_type = request.args.get("parent_type", "")
         parent_id   = request.args.get("parent_id", type=int)
         child_type  = request.args.get("child_type")
+        relation_type = request.args.get("relation_type")
         if not parent_type or parent_id is None:
             return []
-        rels = await get_children(g.s, parent_type, parent_id, child_type)
-        return [
-            {
-                "id":          r.id,
-                "parent_type": r.parent_type,
-                "parent_id":   r.parent_id,
-                "child_type":  r.child_type,
-                "child_id":    r.child_id,
-                "sort_order":  r.sort_order,
-                "label":       r.label,
-            }
-            for r in rels
-        ]
+        rels = await get_children(g.s, parent_type, parent_id, child_type, relation_type=relation_type)
+        return [_serialize_rel(r) for r in rels]
 
     _handlers["get-children"] = _get_children
 
+    # --- get-parents ---
+    async def _get_parents():
+        """Return ContainerRelation parents for a child."""
+        from shared.services.relationships import get_parents
+
+        child_type = request.args.get("child_type", "")
+        child_id = request.args.get("child_id", type=int)
+        parent_type = request.args.get("parent_type")
+        relation_type = request.args.get("relation_type")
+        if not child_type or child_id is None:
+            return []
+        rels = await get_parents(g.s, child_type, child_id, parent_type, relation_type=relation_type)
+        return [_serialize_rel(r) for r in rels]
+
+    _handlers["get-parents"] = _get_parents
+
     return bp
+
+
+def _serialize_rel(r):
+    """Serialize a ContainerRelation to a dict."""
+    return {
+        "id": r.id,
+        "parent_type": r.parent_type,
+        "parent_id": r.parent_id,
+        "child_type": r.child_type,
+        "child_id": r.child_id,
+        "sort_order": r.sort_order,
+        "label": r.label,
+        "relation_type": r.relation_type,
+        "metadata": r.metadata_,
+    }