From f0f2bde789259d6b82fe87daefdd3e2720db28bf Mon Sep 17 00:00:00 2001
From: Connor McArthur <cmcarthur@rjmetrics.com>
Date: Thu, 2 Mar 2017 21:16:41 -0500
Subject: [PATCH 1/3] detect cycles at end of compilation

---
 dbt/compilation.py       |  5 +++++
 dbt/linker.py            | 12 ++++++++++++
 test/unit/test_linker.py | 16 ++++++++++++++++
 3 files changed, 33 insertions(+)

diff --git a/dbt/compilation.py b/dbt/compilation.py
index 7feb0e4edd7..e7fc5bce45d 100644
--- a/dbt/compilation.py
+++ b/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):
diff --git a/dbt/linker.py b/dbt/linker.py
index a56dc29777b..d7c3765c6d0 100644
--- a/dbt/linker.py
+++ b/dbt/linker.py
@@ -28,6 +28,18 @@ 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)
diff --git a/test/unit/test_linker.py b/test/unit/test_linker.py
index 9b9b77cb8d2..0cbf81645a2 100644
--- a/test/unit/test_linker.py
+++ b/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())

From 5648af63e61ed4560fdef0c6c78825ce75b71d77 Mon Sep 17 00:00:00 2001
From: Connor McArthur <cmcarthur@rjmetrics.com>
Date: Thu, 2 Mar 2017 21:19:21 -0500
Subject: [PATCH 2/3] actually stop looking for cycles in loop

---
 dbt/linker.py | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/dbt/linker.py b/dbt/linker.py
index d7c3765c6d0..0fe716ecbb6 100644
--- a/dbt/linker.py
+++ b/dbt/linker.py
@@ -39,7 +39,6 @@ def find_cycles(self):
 
         return None
 
-
     def as_topological_ordering(self, limit_to=None):
         try:
             return nx.topological_sort(self.graph, nbunch=limit_to)
@@ -115,11 +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)
 

From 4504d2395b1d10e4106f23bd564ec4e764f24e14 Mon Sep 17 00:00:00 2001
From: Connor McArthur <cmcarthur@rjmetrics.com>
Date: Thu, 2 Mar 2017 23:26:11 -0500
Subject: [PATCH 3/3] changelog

---
 CHANGELOG.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index a01d69138d1..a66a3e98b7d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@
 ### Changes
 
 - Graph refactor: fix common issues with load order ([#292](https://github.com/fishtown-analytics/dbt/pull/292))
+- Speedup: detect cycles at the end of compilation ([#307](https://github.com/fishtown-analytics/dbt/pull/307))
 
 ## dbt 0.7.1 (February 28, 2017)