From c66bf0ad5695ef547c7f602e5c8a4607ce4699a0 Mon Sep 17 00:00:00 2001
From: Philipp Rudiger <P.Rudiger@ed.ac.uk>
Date: Mon, 11 Dec 2017 22:02:49 +0000
Subject: [PATCH] Ensure Compositor does not make superfluous copies

---
 holoviews/core/options.py   |  5 +++--
 tests/testbokehcallbacks.py | 17 ++++++++++++++---
 2 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/holoviews/core/options.py b/holoviews/core/options.py
index 83e4bb8100..7f3c35d1df 100644
--- a/holoviews/core/options.py
+++ b/holoviews/core/options.py
@@ -880,9 +880,10 @@ def map(cls, obj, mode='data', backend=None):
         if overlay_compositors:
             obj = obj.map(lambda obj: cls.collapse_element(obj, mode=mode, backend=backend),
                           [CompositeOverlay])
-        if element_compositors:
+        element_patterns = [c.pattern for c in element_compositors]
+        if element_compositors and obj.traverse(lambda x: x, element_patterns):
             obj = obj.map(lambda obj: cls.collapse_element(obj, mode=mode, backend=backend),
-                          [c.pattern for c in element_compositors])
+                          element_patterns)
         return obj
 
 
diff --git a/tests/testbokehcallbacks.py b/tests/testbokehcallbacks.py
index dae0b8e7a5..102918f1ca 100644
--- a/tests/testbokehcallbacks.py
+++ b/tests/testbokehcallbacks.py
@@ -1,12 +1,14 @@
 from unittest import SkipTest
 from nose.plugins.attrib import attr
 
-from holoviews.element.comparison import ComparisonTestCase
+from holoviews.core import NdOverlay
 from holoviews.core.options import Store
-
+from holoviews.element import Curve
+from holoviews.element.comparison import ComparisonTestCase
+from holoviews.streams import Selection1D
 
 try:
-    from holoviews.plotting.bokeh.callbacks import Callback
+    from holoviews.plotting.bokeh.callbacks import Callback, Selection1DCallback
     from holoviews.plotting.bokeh.util import bokeh_version
 
     from bokeh.events import Tap
@@ -52,6 +54,15 @@ def test_customjs_callback_attributes_js_for_cb_data(self):
                 'data["y1"] = cb_data["geometry"]["y1"];\n')
         self.assertEqual(js_code, code)
 
+
+    def test_callback_on_ndoverlay_is_attached(self):
+        ndoverlay = NdOverlay({i: Curve([i]) for i in range(5)})
+        selection = Selection1D(source=ndoverlay)
+        plot = bokeh_renderer.get_plot(ndoverlay)
+        self.assertEqual(len(plot.callbacks), 1)
+        self.assertIsInstance(plot.callbacks[0], Selection1DCallback)
+        
+
 @attr(optional=1)
 class TestBokehServerJSCallbacks(ComparisonTestCase):