refactor: Add method to get root of group node in Graph class

src/backend/base/langflow/graph/graph/base.py

@@ -748,6 +748,21 @@ class Graph:
         except KeyError:
             raise ValueError(f"Vertex {vertex_id} not found")
 
+    def get_root_of_group_node(self, vertex_id: str) -> Vertex:
+        """Returns the root of a group node."""
+        if vertex_id in self.top_level_vertices:
+            # Get all vertices with vertex_id as .parent_node_id
+            # then get the one at the top
+            vertices = [vertex for vertex in self.vertices if vertex.parent_node_id == vertex_id]
+            # Now go through successors of the vertices
+            # and get the one that none of its successors is in vertices
+            for vertex in vertices:
+                successors = self.get_all_successors(vertex, recursive=False)
+                if not any(successor in vertices for successor in successors):
+                    return vertex
+        else:
+            raise ValueError(f"Vertex {vertex_id} is not a top level vertex")
+
     async def build_vertex(
         self,
         lock: asyncio.Lock,
@@ -1127,7 +1142,6 @@ class Graph:
         # Initial setup
         visited = set()  # To keep track of visited vertices
         excluded = set()  # To keep track of vertices that should be excluded
-        stack = [vertex_id]  # Use a list as a stack for DFS
 
         def get_successors(vertex, recursive=True):
             # Recursively get the successors of the current vertex
@@ -1143,7 +1157,12 @@ class Graph:
                 successors_result.append(successor)
             return successors_result
 
-        stop_or_start_vertex = self.get_vertex(vertex_id)
+        try:
+            stop_or_start_vertex = self.get_vertex(vertex_id)
+            stack = [vertex_id]  # Use a list as a stack for DFS
+        except ValueError:
+            stop_or_start_vertex = self.get_root_of_group_node(vertex_id)
+            stack = [stop_or_start_vertex.id]
         stop_predecessors = [pre.id for pre in stop_or_start_vertex.predecessors]
         # DFS to collect all vertices that can reach the specified vertex
         while stack: