detect cycles at end of compilation

dbt/compilation.py

@@ -464,6 +464,11 @@ def compile_nodes(self, linker, nodes, macro_generator):
                         "dependency {} not found in graph!".format(
                             dependency))
 
+        cycle = linker.find_cycles()
+
+        if cycle:
+            raise RuntimeError("Found a cycle: {}".format(cycle))
+
         return wrapped_nodes, written_nodes
 
     def generate_macros(self, all_macros):

dbt/linker.py

@@ -28,6 +28,17 @@ def run_type(self):
     def get_node(self, node):
         return self.graph.node[node]
 
+    def find_cycles(self):
+        try:
+            cycles = nx.algorithms.find_cycle(self.graph)
+        except nx.exception.NetworkXNoCycle:
+            return None
+
+        if cycles:
+            return " --> ".join([".".join(node) for node in cycles])
+
+        return None
+
     def as_topological_ordering(self, limit_to=None):
         try:
             return nx.topological_sort(self.graph, nbunch=limit_to)
@@ -103,11 +114,6 @@ def dependency(self, node1, node2):
         self.graph.add_node(node2)
         self.graph.add_edge(node2, node1)
 
-        if len(list(nx.simple_cycles(self.graph))) > 0:
-            raise ValidationException(
-                "Detected a cycle when adding dependency from {} to {}"
-                .format(node1, node2))
-
     def add_node(self, node):
         self.graph.add_node(node)
 

test/unit/test_linker.py

@@ -70,3 +70,19 @@ def test_linker_bad_limit_throws_runtime_error(self):
             self.linker.dependency(l, r)
 
         self.assertRaises(RuntimeError, self.linker.as_dependency_list, ['ZZZ'])
+
+    def test__find_cycles__cycles(self):
+        actual_deps = [('A', 'B'), ('B', 'C'), ('C', 'A')]
+
+        for (l, r) in actual_deps:
+            self.linker.dependency(l, r)
+
+        self.assertIsNotNone(self.linker.find_cycles())
+
+    def test__find_cycles__no_cycles(self):
+        actual_deps = [('A', 'B'), ('B', 'C'), ('C', 'D')]
+
+        for (l, r) in actual_deps:
+            self.linker.dependency(l, r)
+
+        self.assertIsNone(self.linker.find_cycles())