Fixed #17328 -- Added OpenLayersWidget _has_changed method

Thanks Will Hardy for the report and the patch.

AUTHORS

@@ -230,6 +230,7 @@ answer newbie questions, and generally made Django that much better:
     Scot Hacker <shacker@birdhouse.org>
     dAniel hAhler
     hambaloney
+    Will Hardy <django@willhardy.com.au>
     Brian Harring <ferringb@gmail.com>
     Brant Harris
     Ronny Haryanto <http://ronny.haryan.to/>

django/contrib/gis/admin/widgets.py

@@ -4,7 +4,7 @@ from django.templatetags.static import static
 from django.utils import translation
 
 from django.contrib.gis.gdal import OGRException
-from django.contrib.gis.geos import GEOSGeometry, GEOSException
+from django.contrib.gis.geos import GEOSGeometry, GEOSException, fromstr
 
 # Creating a template context that contains Django settings
 # values needed by admin map templates.
@@ -104,3 +104,25 @@ class OpenLayersWidget(Textarea):
                     raise TypeError
                 map_options[js_name] = value
         return map_options
+
+    def _has_changed(self, initial, data):
+        """ Compare geographic value of data with its initial value. """
+
+        # Ensure we are dealing with a geographic object
+        if isinstance(initial, basestring):
+            try:
+                initial = GEOSGeometry(initial)
+            except (GEOSException, ValueError):
+                initial = None
+
+        # Only do a geographic comparison if both values are available
+        if initial and data:
+            data = fromstr(data)
+            data.transform(initial.srid)
+            # If the initial value was not added by the browser, the geometry
+            # provided may be slightly different, the first time it is saved.
+            # The comparison is done with a very low tolerance.
+            return not initial.equals_exact(data, tolerance=0.000001)
+        else:
+            # Check for change of state of existence
+            return bool(initial) != bool(data)

django/contrib/gis/tests/geoadmin/tests.py

@@ -2,7 +2,7 @@ from __future__ import absolute_import
 
 from django.test import TestCase
 from django.contrib.gis import admin
-from django.contrib.gis.geos import Point
+from django.contrib.gis.geos import GEOSGeometry, Point
 
 from .models import City
 
@@ -10,7 +10,7 @@ from .models import City
 class GeoAdminTest(TestCase):
     urls = 'django.contrib.gis.tests.geoadmin.urls'
 
-    def test01_ensure_geographic_media(self):
+    def test_ensure_geographic_media(self):
         geoadmin = admin.site._registry[City]
         admin_js = geoadmin.media.render_js()
         self.assertTrue(any([geoadmin.openlayers_url in js for js in admin_js]))
@@ -33,3 +33,21 @@ class GeoAdminTest(TestCase):
         self.assertIn(
             """geodjango_point.layers.base = new OpenLayers.Layer.WMS("OpenLayers WMS", "http://vmap0.tiles.osgeo.org/wms/vmap0", {layers: \'basic\', format: 'image/jpeg'});""",
             result)
+
+    def test_olwidget_has_changed(self):
+        """ Check that changes are accurately noticed by OpenLayersWidget. """
+        geoadmin = admin.site._registry[City]
+        form = geoadmin.get_changelist_form(None)()
+        has_changed = form.fields['point'].widget._has_changed
+
+        initial = Point(13.4197458572965953, 52.5194108501149799, srid=4326)
+        data_same = "SRID=3857;POINT(1493879.2754093995 6894592.019687599)"
+        data_almost_same = "SRID=3857;POINT(1493879.2754093990 6894592.019687590)"
+        data_changed = "SRID=3857;POINT(1493884.0527237 6894593.8111804)"
+
+        self.assertTrue(has_changed(None, data_changed))
+        self.assertTrue(has_changed(initial, ""))
+        self.assertFalse(has_changed(None, ""))
+        self.assertFalse(has_changed(initial, data_same))
+        self.assertFalse(has_changed(initial, data_almost_same))
+        self.assertTrue(has_changed(initial, data_changed))