Return hash instead of nil when no changes in snapshot

https://trello.com/c/FVI4JXBI

Before this change, attempting to send a snapshot to Slack when there
were no alert or improved constants would return nil, which is an
unexpected value in the app. This change checks to see if those changes
exist before attempting to send a snapshot, and if they don't, returns
a hash with ignored: true instead of nil.

Author: Melanie Gilman

lib/cc/formatters/snapshot_formatter.rb

@@ -59,6 +59,10 @@ def initialize(payload)
         @improved_constants_payload = data.merge("constants" => improved_constants) if improved_constants.any?
       end
 
+      def changed?
+        alert_constants_payload || improved_constants_payload
+      end
+
     private
 
       def new_constants_selector

lib/cc/services/slack.rb

@@ -23,7 +23,13 @@ def receive_test
   end
 
   def receive_snapshot
-    send_snapshot_to_slack(CC::Formatters::SnapshotFormatter::Base.new(payload))
+    snapshot = CC::Formatters::SnapshotFormatter::Base.new(payload)
+
+    if snapshot.changed?
+      send_snapshot_to_slack(snapshot)
+    else
+      { ok: false, ignored: true, message: "No changes in snapshot" }
+    end
   end
 
   def receive_coverage

test/formatters/snapshot_formatter_test.rb

@@ -44,4 +44,16 @@ def test_snapshot_formatter_test_with_relaxed_constraints
     refute_nil f.alert_constants_payload
     refute_nil f.improved_constants_payload
   end
+
+  def test_changed_when_snapshot_changed
+    f = described_class.new({"new_constants" => [],
+                             "changed_constants" => [{"to" => {"rating" => "A"}, "from" => {"rating" => "D"}}]
+    })
+    assert f.changed?
+  end
+
+  def test_changed_when_no_changes
+    f = described_class.new({"new_constants" => [], "changed_constants" => []})
+    refute f.changed?
+  end
 end

test/slack_test.rb

@@ -184,6 +184,17 @@ def test_receive_test
     assert_equal "Test message sent", response[:message]
   end
 
+  def test_no_changes_in_snapshot
+    data = { "name" => "snapshot", "repo_name" => "Rails",
+             "new_constants" => [],
+             "changed_constants" => [],
+    }
+    response = receive_event(data)
+
+    assert_equal false, response[:ok]
+    assert response[:ignored]
+  end
+
   private
 
   def assert_slack_receives(color, event_data, expected_body)