Fixed #28659 -- Fixed LayerMapping crash with null geometry and unique.

django/contrib/gis/utils/layermapping.py

@@ -558,10 +558,14 @@ class LayerMapping:
                             # one from the kwargs WKT, adding in additional
                             # geometries, and update the attribute with the
                             # just-updated geometry WKT.
-                            geom = getattr(m, self.geom_field).ogr
-                            new = OGRGeometry(kwargs[self.geom_field])
-                            for g in new:
-                                geom.add(g)
+                            geom_value = getattr(m, self.geom_field)
+                            if geom_value is None:
+                                geom = OGRGeometry(kwargs[self.geom_field])
+                            else:
+                                geom = geom_value.ogr
+                                new = OGRGeometry(kwargs[self.geom_field])
+                                for g in new:
+                                    geom.add(g)
                             setattr(m, self.geom_field, geom.wkt)
                         except ObjectDoesNotExist:
                             # No unique model exists yet, create.

tests/gis_tests/layermap/models.py

@@ -17,7 +17,7 @@ class State(NamedModel):
 
 class County(NamedModel):
     state = models.ForeignKey(State, models.CASCADE)
-    mpoly = models.MultiPolygonField(srid=4269)  # Multipolygon in NAD83
+    mpoly = models.MultiPolygonField(srid=4269, null=True)  # Multipolygon in NAD83
 
 
 class CountyFeat(NamedModel):

tests/gis_tests/layermap/tests.py

@@ -310,6 +310,17 @@ class LayerMapTest(TestCase):
         self.assertEqual(City.objects.count(), 1)
         self.assertEqual(City.objects.all()[0].name, "Zürich")
 
+    def test_null_geom_with_unique(self):
+        """LayerMapping may be created with a unique and a null geometry."""
+        State.objects.bulk_create([State(name='Colorado'), State(name='Hawaii'), State(name='Texas')])
+        hw = State.objects.get(name='Hawaii')
+        hu = County.objects.create(name='Honolulu', state=hw, mpoly=None)
+        lm = LayerMapping(County, co_shp, co_mapping, transform=False, unique='name')
+        lm.save(silent=True, strict=True)
+        hu.refresh_from_db()
+        self.assertIsNotNone(hu.mpoly)
+        self.assertEqual(hu.mpoly.ogr.num_coords, 449)
+
 
 class OtherRouter:
     def db_for_read(self, model, **hints):