Detecting loops in Sankeys (#271)

* Detecting loops in Sankeys

* Fixing the algo

caravel/viz.py

@@ -1043,7 +1043,32 @@ def query_obj(self):
     def get_data(self):
         df = self.get_df()
         df.columns = ['source', 'target', 'value']
-        return df.to_dict(orient='records')
+        recs = df.to_dict(orient='records')
+
+        hierarchy = defaultdict(set)
+        for row in recs:
+            hierarchy[row['source']].add(row['target'])
+
+        def find_cycle(g):
+            """Whether there's a cycle in a directed graph"""
+            path = set()
+            def visit(vertex):
+                path.add(vertex)
+                for neighbour in g.get(vertex, ()):
+                    if neighbour in path or visit(neighbour):
+                        return (vertex, neighbour)
+                path.remove(vertex)
+            for v in g:
+                cycle = visit(v)
+                if cycle:
+                    return cycle
+
+        cycle = find_cycle(hierarchy)
+        if cycle:
+            raise Exception(
+                "There's a loop in your Sankey, please provide a tree. "
+                "Here's a faulty link: {}".format(cycle))
+        return recs
 
 
 class DirectedForceViz(BaseViz):